handle multipart fields with colon inside #3404

if a form is submitted as multipart/form-data and a field in this form has a colon in its name
e.g. "message_id:signature", the original parameter name would be lost and translated to "message_idsignature"

New pull request of 
https://github.com/playframework/playframework/pull/3404
because I don't know how to modify an existing pull request.