fix tracing request body if source is a function

src/ibrowse_http_client.erl

@@ -1926,5 +1926,7 @@ trace_request_body(Body) ->
             ok
     end.
 
+to_binary({X, _}) when is_function(X) -> to_binary(X);
+to_binary(X) when is_function(X) -> <<"body generated by function">>;
 to_binary(X) when is_list(X)   -> list_to_binary(X);
 to_binary(X) when is_binary(X) -> X.