Always convert s3_protocol to string first

Fixes thoughtbot#883

lib/paperclip/storage/s3.rb

@@ -278,9 +278,9 @@ def s3_permissions(style = default_style)
 
       def s3_protocol(style = default_style)
         protocol = if @s3_protocol.respond_to?(:call)
-          @s3_protocol.call(style, self)
+          @s3_protocol.call(style, self).to_s
         else
-          @s3_protocol
+          @s3_protocol.to_s
         end
 
         protocol = protocol.split(":").first + ":" unless protocol.empty?

test/storage/s3_test.rb

@@ -144,6 +144,22 @@ def teardown
     end
   end
 
+  context ":s3_protocol => :https" do
+    setup do
+      rebuild_model :storage => :s3,
+                    :s3_credentials => {},
+                    :s3_protocol => :https,
+                    :bucket => "bucket",
+                    :path => ":attachment/:basename.:extension"
+      @dummy = Dummy.new
+      @dummy.avatar = StringIO.new(".")
+    end
+
+    should "return a url based on an S3 path" do
+      assert_match %r{^https://s3.amazonaws.com/bucket/avatars/stringio.txt}, @dummy.avatar.url
+    end
+  end
+
   context ":s3_protocol => ''" do
     setup do
       rebuild_model :storage => :s3,