[Issue #2382] AFURLResponseSerializer MIME Type validation fails when using custom/camel-case content types

Added helper method to AFURLResponseSerializer > validateResponse:data:error: for case-insensitive check if acceptableContentTypes contains [response MIMEType]. validateResponse:data:error: would return responseIsValid=NO if custom camel-case content types were used as [response MIMEType] returns the lower-case only version of the MIME type.