Filter the request params through filterParams if it is defined on RESTAdapter

Hey, 

This runs the request params through filterRequest if it is defined. filterRequest must return a modified (or not) params.

I'm not sure if this is the 'correct' way to go about this, however I found it invaluable when getting my ember app talking nicely with my php silex API, which needed various hacks to play nicely with PHP  *cough*.

I tried to get some tests, but I couldnt get the RESTAdapter extended with filterParams in the test environment, so here it is if it is helpful.