[7.x] Add HTTP client assertions assertNotSent() and assertNothingSent()

[christophrumpel]

This PR adds a new assertion to the new HTTP client called assertNotSent.

Similar to the given helper method assertSent, this new one tests the opposite: if a specific request was not sent.

Here is an example:

Http::assertNotSent(function ($request) {
    return $request->hasHeader('X-First', 'foo') &&
           $request->url() == 'http://test.com/users' &&
           $request['name'] == 'Taylor' &&
           $request['role'] == 'Developer';
});

Since there are no specific tests for the assertion helpers, I added one to the HTTP client test file.

[christophrumpel]

After @nunomaduro's feedback, I added another assertion to make sure no request was sent at all.

this->factory->assertNothingSent();

[nuernbergerA]

Nice PR!

[taylorotwell]

Thanks. Probably worth a laravel/docs PR.

[christophrumpel]

Thanks and no problem. Will do a docs PR as well 👍