avoid invalid use of buffer_overflow_handler in the example

FluentHandler does receive buffer_overflow_handler, and its value
should be a callable.
Since logging.Handler example already has handler module in the scope,
it will construct FluentHandler with meaningless buffer overflow
handler (TypeError is raised and ignored).
This change renames the example handler and cause NameError if there
is no handler definition.

Author: yoichi

README.rst

@@ -167,12 +167,12 @@ You can inject your own custom proc to handle buffer overflow in the event of co
     import msgpack
     from io import BytesIO
 
-    def handler(pendings):
+    def overflow_handler(pendings):
         unpacker = msgpack.Unpacker(BytesIO(pendings))
         for unpacked in unpacker:
             print(unpacked)
 
-    logger = sender.FluentSender('app', host='host', port=24224, buffer_overflow_handler=handler)
+    logger = sender.FluentSender('app', host='host', port=24224, buffer_overflow_handler=overflow_handler)
 
 You should handle any exception in handler. fluent-logger ignores exceptions from ``buffer_overflow_handler``.
 
@@ -198,7 +198,7 @@ module.
 
     logging.basicConfig(level=logging.INFO)
     l = logging.getLogger('fluent.test')
-    h = handler.FluentHandler('app.follow', host='host', port=24224, buffer_overflow_handler=handler)
+    h = handler.FluentHandler('app.follow', host='host', port=24224, buffer_overflow_handler=overflow_handler)
     formatter = handler.FluentRecordFormatter(custom_format)
     h.setFormatter(formatter)
     l.addHandler(h)
@@ -228,7 +228,7 @@ You can inject your own custom proc to handle buffer overflow in the event of co
     import msgpack
     from io import BytesIO
 
-    def handler(pendings):
+    def overflow_handler(pendings):
         unpacker = msgpack.Unpacker(BytesIO(pendings))
         for unpacked in unpacker:
             print(unpacked)
@@ -264,7 +264,7 @@ A sample configuration ``logging.yaml`` would be:
                 host: localhost
                 port: 24224
                 tag: test.logging
-                buffer_overflow_handler: handler
+                buffer_overflow_handler: overflow_handler
                 formatter: fluent_fmt
                 level: DEBUG
             none: