allowing generation of presigned URL with a specified content type

I would like to use your library to put images into s3 using presigned URLs and specify that the content-type is image/png or image/jpeg rather than the default of binary/octet-stream.

To allow the content-type to be set in a PUT or POST to s3 it must also be specified (and match) the content-type specified when generating the presigned URL otherwise a 403 forbidden code is returned. 

This change allows the content-type to be set, or if not set behaves as before.