Normalize Content-Length into a single value

If duplicated exactly when merged into a comma seperated list (the
unmerged case is handled already). This is allowed as per RFC 7230
Section 3.3.2.

Author: pgjones

h11/_headers.py

@@ -132,13 +132,16 @@ def normalize_and_validate(headers, _parsed=False):
         raw_name = name
         name = name.lower()
         if name == b"content-length":
+            lengths = set(length.strip() for length in value.split(b","))
+            if len(lengths) != 1:
+                raise LocalProtocolError("conflicting Content-Length headers")
+            value = lengths.pop()
             validate(_content_length_re, value, "bad Content-Length")
             if seen_content_length is None:
                 seen_content_length = value
                 new_headers.append((raw_name, name, value))
-            else:
-                if seen_content_length != value:
-                    raise LocalProtocolError("conflicting Content-Length headers")
+            elif seen_content_length != value:
+                raise LocalProtocolError("conflicting Content-Length headers")
         elif name == b"transfer-encoding":
             # "A server that receives a request message with a transfer coding
             # it does not understand SHOULD respond with 501 (Not

h11/tests/test_headers.py

@@ -57,10 +57,15 @@ def test_normalize_and_validate():
     assert normalize_and_validate(
         [("Content-Length", "0"), ("Content-Length", "0")]
     ) == [(b"content-length", b"0")]
+    assert normalize_and_validate([("Content-Length", "0 , 0")]) == [
+        (b"content-length", b"0")
+    ]
     with pytest.raises(LocalProtocolError):
         normalize_and_validate(
             [("Content-Length", "1"), ("Content-Length", "1"), ("Content-Length", "2")]
         )
+    with pytest.raises(LocalProtocolError):
+        normalize_and_validate([("Content-Length", "1 , 1,2")])
 
     # transfer-encoding
     assert normalize_and_validate([("Transfer-Encoding", "chunked")]) == [