Make s2b/b2s fail silently if the type is wrong.

These functions throw TypeError exceptions if the parameters are already
encoded/decoded. This can be avoided by type checking before attempting
to encode/decode the parameters.

websocket.py

@@ -26,8 +26,8 @@ if sys.hexversion > 0x3000000:
     from io import StringIO
     from http.server import SimpleHTTPRequestHandler
     from urllib.parse import urlsplit
-    b2s = lambda buf: buf.decode('latin_1')
-    s2b = lambda s: s.encode('latin_1')
+    b2s = lambda buf: buf.decode('latin_1') if isinstance(buf, bytes) else buf
+    s2b = lambda s: s.encode('latin_1') if isinstance(s, str) else s
 else:
     # python 2.X
     from cStringIO import StringIO