Update symbol table every time a model is updated. Fixes #196.
Use locally defined conent-type header when referencing a model. Fixes #195.
Recognize ill-formated headers. Fixes #198.
Use `signature` instead of `subjectFirstLine`

Some consistency is always nice however, I would say this should be `signatureBuffer` rather than just a `signature` (as signature is of %PayloadSignature type).
Model content-type header behavior update.

The behavior is as follows:

- Model and referencing payload has specified Content-Type -> Use model's & warn
- Model has specified Content-Type and referencing payload does not -> Use model's
- Model does not have specified Content-Type and referencing payload does -> Use referencing payload's
Merge pull request #197 from apiaryio/zdne/model-regression-fixes

Resource model regression fixes