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
This test is different to the other one - the other one trims the body before sending it.  The other test actually tests a difference between how IE sends multipart/form-data (I can't remember exactly what), but without that test we could easily end up with a regression.