fix validation for consumes/produces with null content-types

### What is the problem / feature ?

Validation of produces/consumes would fail on null content types.

### How did it get fixed / implemented ?

Skip validation if the content-type is null.

### How can someone test / see it ?

Validate a request/response with a null content-type

*Here is a cute animal picture for your troubles...*

![1248874-bigthumbnail](https://cloud.githubusercontent.com/assets/824194/5467050/8dafc1de-8575-11e4-947b-c318c87daf59.jpg)