[mux] Fix _encode_number() in mux_client_for_testing.py
[mux] Fix _encode_number() in mux_client_for_testing.py

The msb of the first byte of number field should be zero as of -05
draft.

Review URL: https://codereview.appspot.com/6554055

git-svn-id: d23f85197b9dafcb1b1682eb9d0eda70517bdc52@668 4ff78f4a-9131-11de-b045-6380ec9940d4

bpsinc-native/src_third_party_pywebsocket_src,bpsinc-native/src_third_party_pywebsocket_src,bpsinc-native/src_third_party_pywebsocket_src,bpsinc-native/src_third_party_pywebsocket_src