See #742 for deprecations.

Before making this change, I did scroll through many pages of github search results looking at existing usage, and it all falls into something better expressed as, e.g.

```
render_hash_as_hidden_fields(params_for_search().except(:q, :search_field, :qt, :page, :utf8))
 ```

I actually couldn't find examples of `:omit_keys` with value-dependent changes (where the :omit_keys feature might actually be useful). If you know of one, let me know.. I think the &block form will satisfy that use case, but it'd be good to see how it works in the real world.