A test to demonstrate a misleading error for chunked response

If a chunked response is truncated exactly before sending a new chunk
(that is, before the first byte of the chunk size), urllib3 throws
`InvalidChunkLength(got length b'\\n', 0 bytes read)`. This is
confusing as no chunk length was sent (a valid chunk length line would
also be \r\n-terminated and we didn't see those either).

In my opinion it would be better if urllib3 threw an IncompleteRead
error instead.

I've worked up a test that demonstrates the problem but I haven't
figured out the right fix yet. I hope this is at least helpful.

test/test_response.py

@@ -1020,6 +1020,22 @@ def test_incomplete_chunk(self) -> None:
         orig_ex = ctx.value.args[1]
         assert isinstance(orig_ex, httplib_IncompleteRead)
 
+    def test_truncated_before_chunk(self) -> None:
+        stream = [b""]
+        fp = MockChunkedNoChunks(stream)
+        r = httplib.HTTPResponse(MockSock)  # type: ignore[arg-type]
+        r.fp = fp  # type: ignore[assignment]
+        r.chunked = True
+        r.chunk_left = None
+        resp = HTTPResponse(
+            r, preload_content=False, headers={"transfer-encoding": "chunked"}
+        )
+        with pytest.raises(ProtocolError) as ctx:
+            next(resp.read_chunked())
+
+        orig_ex = ctx.value.args[1]
+        assert isinstance(orig_ex, IncompleteRead)
+
     def test_invalid_chunk_length(self) -> None:
         stream = [b"foooo", b"bbbbaaaaar"]
         fp = MockChunkedInvalidChunkLength(stream)
@@ -1300,6 +1316,11 @@ def _encode_chunk(self, chunk: bytes) -> bytes:
         return f"{len(chunk):X};asd=qwe\r\n{chunk.decode()}\r\n".encode()
 
 
+class MockChunkedNoChunks(MockChunkedEncodingResponse):
+    def _encode_chunk(self, chunk: bytes) -> bytes:
+        return b""
+
+
 class MockSock:
     @classmethod
     def makefile(cls, *args: typing.Any, **kwargs: typing.Any) -> None: