Return nil when the body is stubbed as '' or nil.

When a real Net::HTTP request is made and receives an
empty-body response (such as a 204 No Content), then the
response body is nil.

This gist demonstrates the behavior:
https://gist.github.com/2918173
Merge branch myronmarston/nil_response_body, closing #32

* myronmarston/nil_response_body:
  Return nil when the body is stubbed as '' or nil.

Conflicts:
	test/test_fake_web.rb
Update test descriptions to reflect intent of 6b2075fae1

Also, use more concise matchers.