Add a check for request body functions and a test for NotImplementedError.

tests/test_custom_request.py

@@ -1,10 +1,17 @@
 from io import BytesIO
 
+import pytest
+
 from sanic import Sanic
 from sanic.request import Request
 from sanic.response import json_dumps, text
 
 
+class DeprecCustomRequest(Request):
+    """Using old API should fail when receive_body is not implemented"""
+    def body_push(self, data):
+        pass
+
 class CustomRequest(Request):
     """Alternative implementation for loading body (non-streaming handlers)"""
     async def receive_body(self):
@@ -13,6 +20,14 @@ class CustomRequest(Request):
             buffer.write(data)
         self.body = buffer.getvalue().upper()
         buffer.close()
+    # Old API may be implemented but won't be used here
+    def body_push(self, data):
+        assert False
+
+
+def test_deprecated_custom_request():
+    with pytest.raises(NotImplementedError):
+        Sanic(request_class=DeprecCustomRequest)
 
 def test_custom_request():
     app = Sanic(request_class=CustomRequest)