Ignore :payload options for Search#params.

When building a Tire.search DSL with a hash of options for th payload, we need to ignore the `:payload` in `params` so the `to_curl` and logging does not include a bunch of noise.

You may want to consider ignore :payload in these two places too. Thanks!

* https://github.com/karmi/tire/blob/master/lib/tire/count.rb#L26
* https://github.com/karmi/tire/blob/master/lib/tire/multi_search.rb#L181