Fix string length computation for issue_command function

When arg is not a ascii char,
len(arg) may be differrent from len(arg.encode("utf-8")).
For example, a Chinese character `c`,
len(c) is 1, but len(c.encode("utf-8")) is 3,
which will raise an InvalidResponseError because of the wrong length.

This commit should fix this kind of problem.

Author: shunyi

webkit_server.py

@@ -514,7 +514,7 @@ def issue_command(self, cmd, *args):
     self._writeline(str(len(args)))
     for arg in args:
       arg = str(arg)
-      self._writeline(str(len(arg)))
+      self._writeline(str(len(arg.encode("utf-8"))))
       self._sock.sendall(arg.encode("utf-8"))
 
     return self._read_response()