Fix file descriptor not closed (prometheus#269)

Signed-off-by: Geoffrey Beausire <g.beausire@criteo.com>

Author: geobeau

prometheus_client/core.py

@@ -426,6 +426,8 @@ def write_value(self, key, value):
 
     def close(self):
         if self._f:
+            self._m.close()
+            self._m = None
             self._f.close()
             self._f = None