Add `post_form' for async

[objmagic]

Resolves #235

[rgrinberg]

1. The original function is very strange to me. Why would ~params be typed as a header? It should simply be a key-value list. Nevermind that Header.t coincides with that now. @avsm ?

2. I don't like the copy pasting between lwt/async. Can we simply have a backend independent Request.url_encoded that will construct the request for lwt/async?

[objmagic]

Yeah, simple copy pasting is so easy to do. I think cohttp needs to find a better way to solve those inconsistencies.

[rgrinberg]

In this particular case pulling out the request construction to Request works good. Still doesn't address 1 however.

[avsm]

iirc, the form handling is vestigial from the original ocaml-http library. I see no reason why it shouldn't be built over the core HTTP library, as it's just a client that adds specific headers and a body format.

[rgrinberg]

@avsm and why is ~params of type Header.t? That seems wrong to me.

[avsm]

@rgrinberg I agree that ~params being a Header.t seems wrong (although it's almost the same data type, with a slightly different encoding, so it's not fully weird, just named wrongly).

[rgrinberg]

@marklrh Can you have a look #257 and make the same change in this PR. Should be good to merge after that.

I've tried to think of a way to get rid of the code duplication but I don't want Request to depend on Body.

[objmagic]

since i deleted the repo days ago, I have to create a new pull at #258