Qualify buffer_copy, don't qualify buffer_size:

fix #1416

* Calls to buffer_copy are qualified as net::buffer_copy
* Calls to buffer_size are made unqualified, permitting
  argument dependent lookup to take effect.

test/doc/exemplars.cpp

@@ -164,7 +164,8 @@ struct BodyReader
         // The specification requires this to indicate "no error"
         ec = {};
 
-        return net::buffer_size(buffers);
+        using net::buffer_size;
+        return buffer_size(buffers);
     }
 
     /** Called when the body is complete.