Updating the ProxyKmipClient to simplify closing

This change updates the ProxyKmipClient close method, allowing
it to be called without error even when the client connection is
not open. The client unit tests have been updated to reflect this.

kmip/pie/client.py

@@ -134,11 +134,10 @@ class ProxyKmipClient(api.KmipClient):
         Close the client connection.
 
         Raises:
-            ClientConnectionNotOpen: if the client connection is not open
             Exception: if an error occurs while trying to close the connection
         """
         if not self._is_open:
-            raise exceptions.ClientConnectionNotOpen()
+            return
         else:
             try:
                 self.proxy.close()