Add header helpers

I wasn't able to write test since I couldn't fix the [Output](https://github.com/Codeception/Codeception/blob/master/src/Codeception/Output.php#L26) file to delay the print so the php headers are in the test script...

Test are fairly easy, I just need to add some headers in the [index.php](https://github.com/Codeception/Codeception/blob/master/tests/data/rest/index.php)

Also I fixed a bad header-merge logic in the Universal Connector.