Header formatting 

Added null check to request header accessor 

Merge pull request php-vcr#1 from thecoderszone/symfony-5 Symfony 5

Merge pull request php-vcr#290 from janvernieuwe/travis-update Test php 7.3 and 7.4

Fix curl reset (php-vcr#248) ### Context When using PHP-VCR with Guzzle 6 with async requests, the first request is playing correctly but the next request is putting Guzzle in an infinite loop. It took me a whole day but I traced back this problem to the way the "curl_reset" hook is written. It clears the request but not the response. Hence, when another request is performed (even on a different URL) the same response is sent. This weird behaviour is causing an infinite loop in Guzzle 6. This problem is probably exactly the same as the issue php-vcr#211. This PR should fix it. ### What has been done The first commit of this PR contains only the failing test (to showcase the problem). You see that the test never completes (because of the infinite loop): https://travis-ci.org/php-vcr/php-vcr/jobs/366140696 The second commit is the fix. I simply added one line in the curl_reset hook to remove the response as well as the request. Note: this PR is based on the branch from PR php-vcr#246 since I need unit tests from Guzzle 6 that are not yet merged.

Merge pull request php-vcr#246 from moufmouf/guzzle6-integration-tests Adding integration tests for Guzzle 6

Merge pull request php-vcr#235 from tiagobrito/Allow-symfony/yaml-v4-… …as-dependency [RFC] Allow symfony/yaml and symfony/event-dispatcher v4.0 as dependency

Merge pull request php-vcr#206 from alnorth/empty-string-post-fix Make handling of empty strings in CURLOPT_POSTFIELDS consistent.

Merge pull request php-vcr#220 from renatomefi/pr/192 CurlHook now returns integer HTTP status codes

Merge pull request php-vcr#218 from renatomefi/fix/pr-215-host-header When setting the Host header check if it's not null