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 +159,11 @@ 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 non-zero: {content_length}"
+                )
 
         return cls(path, headers)