Avoid overwriting request's params with silk params

[volrath]

Ring request's :params were overwritten by ring-handler, which assoc its params in the request.

This scenario might be a common one, specially if the user is using something like ring.middlewares.params.wrap-params, for example.

This PR makes sure silk :params are merged to any possible existing request's :params.

I wrote a test for it, let me know if it's enough or if I should test or improve anything else.

Closes #21

[volrath]

Hmm not sure why that failed. I ran lein dev locally and all test passed. I'm fairly new to Clojure, if you can give me a hint I'll fix it.

[volrath]

done @DomKM