fix: allow 'Content-Length' = "0"

Shelly devices send a 'Content-Length' header with a value of "0".
Allow this as this also means there is not a request body.

In addition give a bit more detail in the error message.

Closes: #1616

Author: JohnVillalovos

src/websockets/http11.py

@@ -159,7 +159,12 @@ def parse(
             raise NotImplementedError("transfer codings aren't supported")
 
         if "Content-Length" in headers:
-            raise ValueError("unsupported request body")
+            content_length = headers["Content-Length"]
+            if content_length != "0":
+                raise ValueError(
+                    f"unsupported request body as 'Content-Length' is "
+                    f"non-zero: {content_length}"
+                )
 
         return cls(path, headers)
 

tests/test_http11.py

@@ -83,7 +83,7 @@ def test_parse_body(self):
             next(self.parse())
         self.assertEqual(
             str(raised.exception),
-            "unsupported request body",
+            "unsupported request body as 'Content-Length' is non-zero: 3",
         )
 
     def test_parse_body_with_transfer_encoding(self):