Issue #269 Unit tests for Request and Response properties of AssertionContext

[m-posluszny]

I've written all required unit tests for success and failure scenarios for AssertionContext needed for #269
Placed it under expect_test.go

I've implemented a new interface for AssertionHandler that stores context state, so later I can use it to validate fields.

[coveralls]

coverage: 94.841% (+0.03%) from 94.813%
when pulling abd980f on m-posluszny:tests-AssertionContext-req-resp
into d0cfaf9 on gavv:master.