MINOR: sample: make the param converter support control characters

MINOR: sample: make the param converter support control characters

Sometimes it can be convenient to support delimiting of the param()
converter using control characters that can be found in some request
bodies. Since the converter only supports a single character, we can
easily make an exception for "0xHH". That's what this patch does. The
reg-test was updated to include one 0x26.