Tweak encoding behaviour

- Don't fail to identity encoding when an unknown encoding is specified.
- Don't constrain encodings. I want to try to modify traffic as little as
possible by default.
- When decoding, delete content-encoding header rather than set it to "identity"
- Refuse to decode/encode when there is an existing but unknown
content-encoding header.

libmproxy/encoding.py

@@ -8,21 +8,25 @@ __ALL__ = ["ENCODINGS"]
 
 ENCODINGS = set(["identity", "gzip", "deflate"])
 
-def decode(encoding, content):
+def decode(e, content):
     encoding_map = {
         "identity": identity,
         "gzip": decode_gzip,
         "deflate": decode_deflate,
     }
-    return encoding_map.get(encoding, identity)(content)
+    if e not in encoding_map:
+        return None
+    return encoding_map[e](content)
 
-def encode(encoding, content):
+def encode(e, content):
     encoding_map = {
         "identity": identity,
         "gzip": encode_gzip,
         "deflate": encode_deflate,
     }
-    return encoding_map.get(encoding, identity)(content)
+    if e not in encoding_map:
+        return None
+    return encoding_map[e](content)
 
 def identity(content):
     """

libmproxy/flow.py

@@ -588,8 +588,6 @@ class FlowMaster(controller.Master):
             f.request.anticache()
         if self.anticomp:
             f.request.anticomp()
-        else:
-            f.request.constrain_encoding()
 
         if self.server_playback:
             pb = self.do_server_playback(f)

libmproxy/proxy.py

@@ -134,11 +134,14 @@ class HTTPMsg(controller.Msg):
             Content-Encoding header and changing Content-Encoding header to
             'identity'.
         """
+        ce = self.headers["content-encoding"]
+        if not ce or ce[0] not in encoding.ENCODINGS:
+            return
         self.content = encoding.decode(
-            (self.headers["content-encoding"] or ["identity"])[0],
+            ce[0],
             self.content
         )
-        self.headers["content-encoding"] = ["identity"]
+        del self.headers["content-encoding"]
 
     def encode(self, e):
         """