Port multipart goodness to to 0.18.x branch

[jmcardon]

:D

[jmcardon]

seems to pass all tests, but trying it on a small client I cannot get it to run properly, so I'm double checking first if there is a problem was bugged

[jmcardon]

Bugs have been fixed, and tested with postman. See notes on #1728.

[rossabaker]

Wait a second, what's the difference between this and #1728 other than target branch? Aren't we just going to roll this one into master once merged?

[jmcardon]

@rossabaker mostly that the #1728 version doesnt deprecate the other stuff for the sake of MiMa, it straight up deletes the old version.

[rossabaker]

Yeah, it's going to have to to get rid of that dependency. I am pro-deprecation until it exerts drag, like pulling in a new dependency.

Okay, merging this one, will roll forward, and will use #1728 as a guideline if the merge is ugly.