Collection.sync without parameters send an undefined Content-Type

https://github.com/inkling/backbone-mootools/blob/master/MooToolsAdapter.js#L320
when params.contentType is not passed it makes Request send an 'undefined' Content-Type.

I changed that to headers: { 'Content-Type': params.contentType || 'application/json' } so my server will not bitch me that undefined is not a proper Content-Type, but it's probably better to not pass Content-Type at all.
