Clear the urllib3.PoolManager pool after completing a test (#314)

This prevents running out of sockets with very large test suites.

Author: scottwallacesh

gabbi/httpclient.py

@@ -51,6 +51,8 @@ def request(self, absolute_uri, method, body, headers, redirect):
         headers = response.headers
         headers['status'] = str(status)
         headers['reason'] = reason
+
+        self.clear()
         return headers, content