Check that custom headers isn't nil before setting

We hit an issue with Ruby 1.9.3 where @custom_headers was being overwritten by a nil hash, and the ZabbixAPI invocation was failing. Passing in :custom_headers => {} works, but it would be better to check that options[:custom_headers] isn't nil before setting it.
Merge pull request #22 from errorstudio/master

Check that custom headers isn't nil before setting