cleaned up HTTPResponse.php

did some quick online search, and it seems `$this` is actually the most appropriate notation here. see: http://www.phpdoc.org/docs/latest/guides/types.html#keywords

> self
> An object of the class where this type was used, if inherited it will still represent the class where it was originally defined.
> 
> static
> An object of the class where this value was consumed, if inherited it will represent the child class. (see late static binding in the PHP manual).
> 
> $this
> This exact object instance, usually used to denote a fluent interface.