Send not processable params

Discussed in https://github.com/rollbar/rollbar-gem/issues/152.

We have two solutions for this, the proposed in this PR or https://github.com/rollbar/rollbar-gem/commit/f33fc0301d1d83da74ddf51fed65ee2df7030de6.

This PR just inspect the payload and sends it i a "can't process params" key.