Fix #4163 - Update merge_check_key to keep up with 2.1.4 change

The merge_check_key method (found in Msf::Module::ModuleInfo)) uses
respond_to? to check is our object includes a merge_info_description
method before merging descriptions. The respond_to? method in 2.1.4
by default no longer checks private and protected methods, and this
is breaking our merge_check_key method.

Fix #4163
Land #4191, merge_check_key update for Ruby 2.1.4