Emit drain event when the request is ready to receive more data

[arnaud-lb]

Currently, HttpClient\Request isn't a well-behaved WritableStreamInterface: write() can return false (telling the user to stop writing to it until it emits a drain event), but there are cases where it never emits a drain event: When calling write() before the headers have been sent, write() returns false and actually writes the body later. However, it doesn't emit a drain event.

This is a problem, for instance, when using pipe():

$request = $client->request("POST", "http://example.com");
Util::pipe($readableStream, $request);

pipe() will pause $readableStream when $request->write() returns false, and resume $readableStream when $request emits a drain event. If $request never emits a drain event, the pipe operation never finishes.

This PR fixes that by emitting a drain event when the request is ready to receive more write() calls.

[WyriHaximus]

Hey thank you for your PR, sounds and looks good at first glance. Going to review it and come back to you later 👍

[clue]

Changes LGTM 👍

[WyriHaximus]

@clue thanks!

@arnaud-lb My apologise for the delay, things didn't go as planned and I'll go over this PR and merge and tag it tomorrow when I don't find anything with it :)

[WyriHaximus]

Thanks again for the PR, I'll tag it shortly

[arnaud-lb]

Thanks @WyriHaximus @clue :)