Fixes #12 Implement response header validation

### What is the problem / feature ?

Response headers were not being validated

### How did it get fixed / implemented ?

Added validation to the response headers.

### How can someone test / see it ?

See that response headers of non-string-types are validated correctly.

*Here is a cute animal picture for your troubles...*

![tongue-polar-bear](https://cloud.githubusercontent.com/assets/824194/5002843/b0e35950-69c2-11e4-8243-665010d1b32b.jpg)