HTTP client: Support passing default request headers

[clue]

Here's what this could potentially look like:

$browser = new React\Http\Browser();
$browser = $browser->withProtocolVersion('1.1'); // already supported
$browser = $browser->withHeader('User-Agent', 'ACME'); // could be useful?

$browser->get($url)->then(…);

This isn't currently supported, but seems reasonable and would also allow other packages to take advantage of custom HTTP request headers. What do you think about this?

[WyriHaximus]

Seeing this pattern in my own projects where I do this for the Authorization header. So lets add this 👍

[bartvanhoutte]

👋 Unsetting a default header, would that happen through

$browser = $browser->withHeader('User-Agent', null);

or

$browser = $browser->withoutHeader('User-Agent')

What do you think?

[clue]

:wave: Unsetting a default header, would that happen through […]

$browser = $browser->withoutHeader('User-Agent')

@bartvanhoutte The idea is definitely to keep this in line with our existing interfaces. In particular, PSR-7 (HTTP messages) already define a withoutHeader() method, so I would absolutely go for this as well here :+1:

[clue]

Closed via #461 :shipit: