Accept empty responses in tests

cgimap and the rails port return an empty response for some error
replies (e.g. 404 and 410). Make the test framework accept this.

If a test is expecting content, the nature of the content is tested
in the checks.

Fixes issues introduced in zerebubuth/openstreetmap-cgimap#26
Merge pull request #29 from pnorman/empty_error_test

Accept empty responses in tests