Added test for reading POST date in RequestInterceptor

Relating to this issue:

https://github.com/lightbody/browsermob-proxy/issues/7

I finally got around to writing a test that fails. Hopefully this helps.

EDIT:

Dammit, i'm still new to Github and obviously i selected the wrong comparison for this pull request. Please disregard :-/.

Also, appreciate some help getting the right one :-) I made a fork and am trying to sue that as the base for comparison,