Pass aditional parameters to S3 expiring urls

This adds functionality to add additional params to the querystring for
S3 expiring URLs. The reason for this is if you want to override
response_content_type or response_content_disposition with expiring
URLs, you have to change Amazon's signature, which gets signed before
adding additional options to the querystring. Because it's added later,
and because Amazon's signature includes the full URL, the signature is
bad and the request fails.

To use this feature:

    has_attached_file :avatar,
                      :s3_url_options => { :response_content_disposition => "inline" }

Additionally, you can pass a lambda and it'll be evaluated when the URL
is generated. If, for example, the content type is incorrect in Amazon
(either empty or application/octet-stream), you can effectively cast the
response from Amazon as a particular content type.

    has_attached_file :avatar,
                      :s3_url_options => lambda {|model| { :response_content_type => model.avatar_content_type } }

lib/paperclip/storage/s3.rb

@@ -141,7 +141,8 @@ module Paperclip
 
       def expiring_url(time = 3600, style_name = default_style)
         if path
-          s3_object(style_name).url_for(:read, :expires => time, :secure => use_secure_protocol?(style_name)).to_s
+          base_options = { :expires => time, :secure => use_secure_protocol?(style_name) }
+          s3_object(style_name).url_for(:read, base_options.merge(s3_url_options)).to_s
         end
       end
 
@@ -159,6 +160,12 @@ module Paperclip
         @s3_host_alias
       end
 
+      def s3_url_options
+        s3_url_options = @options[:s3_url_options] || {}
+        s3_url_options = s3_url_options.call(instance) if s3_url_options.is_a?(Proc)
+        s3_url_options
+      end
+
       def bucket_name
         @bucket = @options[:bucket] || s3_credentials[:bucket]
         @bucket = @bucket.call(self) if @bucket.is_a?(Proc)

test/storage/s3_test.rb

@@ -350,16 +350,24 @@ class S3Test < Test::Unit::TestCase
   end
 
   context "Generating a secure url with an expiration" do
-    setup do
-      rebuild_model :storage => :s3,
-                    :s3_credentials => {
-                      :production   => { :bucket => "prod_bucket" },
-                      :development  => { :bucket => "dev_bucket" }
-                    },
-                    :s3_host_alias => "something.something.com",
-                    :s3_permissions => "private",
-                    :path => ":attachment/:basename.:extension",
-                    :url => ":s3_alias_url"
+    def build_model_with_options(options = {})
+      base_options = {
+        :storage => :s3,
+        :s3_credentials => {
+          :production   => { :bucket => "prod_bucket" },
+          :development  => { :bucket => "dev_bucket" }
+        },
+        :s3_host_alias => "something.something.com",
+        :s3_permissions => "private",
+        :path => ":attachment/:basename.:extension",
+        :url => ":s3_alias_url"
+      }
+
+      rebuild_model base_options.merge(options)
+    end
+
+    should "use default options" do
+      build_model_with_options
 
       rails_env("production")
 
@@ -373,8 +381,40 @@ class S3Test < Test::Unit::TestCase
       @dummy.avatar.expiring_url
     end
 
-    should "should succeed" do
-      assert true
+    should "allow overriding s3_url_options" do
+      build_model_with_options :s3_url_options => { :response_content_disposition => "inline" }
+
+      rails_env("production")
+
+      @dummy = Dummy.new
+      @dummy.avatar = StringIO.new(".")
+
+      object = stub
+      @dummy.avatar.stubs(:s3_object).returns(object)
+      object.expects(:url_for).with(:read, :expires => 3600, :secure => true, :response_content_disposition => "inline")
+
+      @dummy.avatar.expiring_url
+    end
+
+    should "allow overriding s3_object options with a proc" do
+      build_model_with_options :s3_url_options => lambda {|attachment| { :response_content_type => attachment.avatar_content_type } }
+
+      rails_env("production")
+
+      @dummy = Dummy.new
+
+      @file = StringIO.new(".")
+      @file.stubs(:original_filename).returns("5k.png\n\n")
+      @file.stubs(:content_type).returns("image/png\n\n")
+      @file.stubs(:to_tempfile).returns(@file)
+
+      @dummy.avatar = @file
+
+      object = stub
+      @dummy.avatar.stubs(:s3_object).returns(object)
+      object.expects(:url_for).with(:read, :expires => 3600, :secure => true, :response_content_type => "image/png")
+
+      @dummy.avatar.expiring_url
     end
   end