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

@@ -132,6 +132,11 @@ def normalize_and_validate(headers, _parsed=False):
         raw_name = name
         name = name.lower()
         if name == b"content-length":
+            if b"," in value:
+                lengths = set(length.strip() for length in value.split(b","))
+                if len(lengths) != 1:
+                    raise LocalProtocolError("conflicting Content-Length headers")
+                value = list(lengths)[0]
             validate(_content_length_re, value, "bad Content-Length")
             if seen_content_length is None:
                 seen_content_length = value

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")]) == [