Use variable for payload

[ywangd]

This should work for both single and bulk requests

[ywangd]

If variable is used directly at the payload position, this will make it impossible to write compact multiple statements, e.g. the following will not work:

GET /
echo

The above is currently possible because the function name, which is also a variable type, can syntatically signal the end of the API call. If echo can be parsed as the payload, this would likely to cause bugs when writing a script file.