[5.5] Allow default headers to be sent with requests

[mateusjatenee]

This PR allows default headers to be sent with the requests. So, instead of having to override the call method or having to use something as

$this->withServerVariables([
    'HTTP_FOO' => 'bar'
]);

$this->get('/');

You can do

$this->withHeaders([
    'foo' => 'bar'
]);

$this->get('/');

You can also clean the headers with $this->cleanHeaders();

I've been having to send default headers quite a lot on an app and thought it would be cool to have it on the core. let me know what you all think

[DCzajkowski]

I'd like to see a withHeader($name, $value) method too. Or just header($name, $value) to compliment that from Illuminate\Http\ResponseTrait

[taylorotwell]

I would rather see it clear the headers after each test.

[mateusjatenee]

@DCzajkowski in the withHeader case, would it be "global" or only applied in the next request?

@taylorotwell what worries me is if the user needs to clear the headers on the same test, in another request. Any ideas?

Edit:
Have to leave for a bit, but I added a withHeader method @DCzajkowski. Take a look

[taylorotwell]

Wouldn't they just call ->clearHeaders() manually in that case?

[mateusjatenee]

@taylorotwell oh, I thought you were saying you would rather not have the clearHeaders method and just have it reset after each test. I believe the way it is set up it resets after each test? i can test it once I get home

[mateusjatenee]

Added a commit to flush the headers on the tearDown method