Fix failure case spec with gloal author

The issue was that the specs were trying to assert that the git author was not set.  However, even though it was unset in the spec repo dir, it *was* set globally in the `~/.gitconfig`.  This resulted in failing specs.  The solution was to stub out the `duet.env` namespace in the context of the specs.  It was necessary to do this with an environmental variable instead of stubbing because the actual `git` commands run in the integration spec by shelling out use the real executables and lib for this gem, which are not affected by the stubs.