Added tests for passing S3 credentials as a proc.

Fixes #738

lib/paperclip/storage/s3.rb

@@ -234,6 +234,7 @@ module Paperclip
       end
 
       def parse_credentials creds
+        creds = creds.respond_to?('call') ? creds.call(self) : creds
         creds = find_credentials(creds).stringify_keys
         env = Object.const_defined?(:Rails) ? Rails.env : nil
         (creds[env] || creds).symbolize_keys
@@ -333,7 +334,7 @@ module Paperclip
         when Hash
           creds
         else
-          raise ArgumentError, "Credentials are not a path, file, or hash."
+          raise ArgumentError, "Credentials are not a path, file, proc, or hash."
         end
       end
       private :find_credentials

test/storage/s3_test.rb

@@ -618,6 +618,21 @@ class S3Test < Test::Unit::TestCase
     end
   end
 
+  context "An attachment with S3 storage and S3 credentials defined as a Proc" do
+    setup do
+      rebuild_model :storage => :s3,
+                    :bucket => {:not => :important},
+                    :s3_credentials => lambda { |attachment|
+                      Hash['access_key_id' => "access#{attachment.instance.other}", 'secret_access_key' => "secret#{attachment.instance.other}"]
+                    }
+    end
+
+    should "get the right credentials" do
+      assert "access1234", Dummy.new(:other => '1234').avatar.s3_credentials[:access_key_id]
+      assert "secret1234", Dummy.new(:other => '1234').avatar.s3_credentials[:secret_access_key]
+    end
+  end
+  
   context "An attachment with S3 storage and specific s3 headers set" do
     setup do
       rebuild_model :storage => :s3,