handle multipart fields with colon inside [2.4.x]

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"

adjusted pull request for changes made from 2.3.x -> 2.4.x
Original pull request: https://github.com/playframework/playframework/pull/3404
Pull Request on 2.3.x: https://github.com/playframework/playframework/pull/3546