Make configuration of :s3_credentials optional

Since AWS-SDK 1.5.3 [1] the library automatically checks for credentials.
This makes it unnecessary to supply credentials when they are present
in environment variables or supplied via an IAM Instance Profile.

[1] http://aws.amazon.com/releasenotes/Ruby/0304005114813505

lib/paperclip/storage/s3.rb

@@ -366,8 +366,10 @@ module Paperclip
           YAML::load(ERB.new(File.read(creds)).result)
         when Hash
           creds
+        when NilClass
+          {}
         else
-          raise ArgumentError, "Credentials are not a path, file, proc, or hash."
+          raise ArgumentError, "Credentials given are not a path, file, proc, or hash."
         end
       end
 

test/storage/s3_test.rb

@@ -737,6 +737,17 @@ class S3Test < Test::Unit::TestCase
     end
   end
 
+  context "An attachment with S3 storage and S3 credentials not supplied" do
+    setup do
+      rebuild_model :storage => :s3, :bucket => "testing"
+      @dummy = Dummy.new
+    end
+
+    should "not parse any credentials" do
+      assert_equal({}, @dummy.avatar.s3_credentials)
+    end
+  end
+
   context "An attachment with S3 storage and specific s3 headers set" do
     setup do
       rebuild_model :storage => :s3,