Chunk response unit-testing

@3rd-Eden, I'm not sure this is what you _really_ want. But it does takes the parser in test for chunked responses. I've added (a little bit too much) comments on the test to make sure everyone understands what I'm doing. Because it's kinda messed up, haha.

It also required for the `buffer` function to be inside the `memcached` object. Did not found a nice way to access it otherwise (may be overlooking something here?).

Also found a scoping issue in the `makeCallback` erroring that I've fixed + added a unittest for the case that was triggering the crash. 

Comments, shoot! :)