Add type check before generating url

fixes madglory/backbone-fetch-cache#89
Add some tests

fix madglory/backbone-fetch-cache#89
Merge branch 'master' of github.com:GermainBergeron/backbone-fetch-cache into GermainBergeron-master

Conflicts:
	backbone.fetch-cache.min.js
Merge branch 'GermainBergeron-master'