Refactoring of HttpGetTask / HttpRequestTask

* Added unit tests for the tasks
* Moved config / headers / authentication handling to an abstract base `HttpTask`, both `HttpGetTask` and `HttpRequestTask` inherit from that
* This means that `HttpGetTask` can now use fully configured `HTTP_Request2` instance without adding new attributes like `followRedirects` / `sslVerifyPeer` / `proxy`
* It is now possible to set default configuration for all tasks of these types via `phing.http.*` properties (see the tests)

Documentation should be updated, but I didn't test whether docbook build succeeds

Also updated the version of HTTP_Request2 dependency.