Fixes an issue with using Procs as namespaces

The namespacing code was setup to check if the namespace was already a
part of the key and prepend it if not. The problem was that while
ActiveSupport::Cache::Store and ActiveSupport::Cache::RedisStore support
procs the actual Redis::Store did not. I just added a conditional to
check if the namespace was a proc and use the result if so, otherwise
fallback to the original code. I did not add any tests as much of the
namespace tests were commented out.