Use Python 3 str in incoming (#541)

Fixes Python 2 unicode issues.

Supersedes #539 to fix regression in #514

rosbridge_library/src/rosbridge_library/protocol.py

@@ -30,6 +30,9 @@
 # ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 # POSSIBILITY OF SUCH DAMAGE.
 
+from __future__ import unicode_literals
+from builtins import str
+
 import rospy
 import time