make httpClient protected. Try to introduce single entry point for internal execute implementation

[chudzikp]

Two changes. First is protected httpClient so other users should be able to introduce they own logic around execute methods.

Second one is ugly hack :( I've been trying to introduce single point of entry for all execute methods. It works but it's ugly and I have no idea how to make it look better and less hacky but it does its job...

[dkocher]

I will have a look at the test failures.