Fix requirements for unit tests dependening on pecl_http.

Version 2 of the extension no longer ships with the functions we are using. So for people using
pecl_http > 1.7.x a simple "@requires extension http" would return true, but the tests would still
fail. We can't require specific extension versions, so testing for the exact function we are using
is the best we can do.

Unit tests with pecl_http-2.0.7 installed