fix(Request): QUERY_STRING is not required to be present in env

Per PEP-333, QUERY_STRING may be empty or absent. It seems to be absent if using eventlet.wsgi.server so this change checks for
QUERY_STRING in env and behaves as before if present. If absent, this sets query_string to empty string.
fix(Request) forgot to add a test
Pull request feedback fixes
Merge pull request #89 from rs-randallburt/query_string_fix

fix(Request): QUERY_STRING is not required to be present in env