Locally disable rspec warning of message expectations on nil object

I disable the warning only in the context of a single example where
the test deliberately sets the user to nil and set message
expectations on it.

The warnings were:
```
      when current user is nil
An expectation of :present? was set on nil. Called from /home/david2/LocalSupport/spec/controllers/volunteer_ops_controller_spec.rb:121:in `block (5 levels) in <top (required)>'. Use allow_message_expectations_on_nil to disable warnings.
An expectation of :organization was set on nil. Called from /home/david2/LocalSupport/spec/controllers/volunteer_ops_controller_spec.rb:122:in `block (5 levels) in <top (required)>'. Use allow_message_expectations_on_nil to disable warnings.
```
Merge pull request #52 from dcorking/fix-rspec-nil-warning

Disable rspec nil warning