Fixes #8263: Remove usage of to_sym on user input params.

Addresses CVE-2014-3712, whereby two locations in the code turn user
input into symbols and allow potential DoS attacks by an authenticated user.

The first location, content search params, was turned from symbol matching
into string matching to avoid the to_sym conversion. The second location
involves the use of the Rails action param. While this should be guarded
by the internals of Rails, the code was changed to only perform the to_sym
if the params[:action] parameter exists within the application by doing
the respond_to? check prior to the to_sym in the send.
Merge pull request #4802 from ehelms/fixes-8263

Fixes #8263: Remove usage of to_sym on user input params.