Ensure request is defined before calling methods on it

In #is_robot? and #is_ignore_ip_address?, request is assumed to be
accessible. As a result, the Split::Helper module can only really be
included in a Controller context.

This commit removes that requirement by conditionally checking that
request is defined by attempting to call methods on it.
Merge pull request #190 from nfm/remove-dependency-on-request-in-split-helper

Ensure request is defined before calling methods on it