Update to check for pecl_http 2.x.x

Check for http\Heaader namespace version of code, implies newer 2.x.x series of code
Update to global namespace version

we're already in a namespace so need to use absolute namespacing \http\Header
Global namespace version throughout \http\Header
Merge pull request #116 from phpcodemonkey/master

Update to make use of pecl_http 2.x.x series code