Allow http_build_query() override

[battis]

This seems like the kind of thing that one should never want to do. Unless... you're dealing with a RESTful API that doesn't seem to understand URL parameters that contain numeric indices for the base array. For example:

Instructure Canvas is expecting URL parameters for arrays that do not include numerical indices:

http://canvas-instance.instructure.com/api/v1/courses/123/sections?include[]=students&include[]=enrollments

At the same time http_build_query() is trying to build legal URL parameters, which require that at least the base URL be indexed, resulting in shenanigans like this:

http://canvas-instance.instructure.com/api/v1/courses/123/sections?include%5B0%5D=students&include%5B1%5D=enrollments

Adding in an override-able Pest::http_build_query() method allows for child classes to make necessary tweaks to the URL parameters before sending them to the API. For example:

class CanvasPest extends Pest {

  /* ... */

  protected function http_build_query($data) {
    return preg_replace('/%5B\d+%5B/simU', '[]', http_build_query($data));
  }

  /* ... */

}