Set a request preprocessor
A function or expression that modifies
Specifically, a valid input is one of:
A function taking a single argument, the
request, and returning a valid
A formula as shorthand for an anonymous function with
. as the
"request" argument, as in the
A list of functions/formulas, which will be executed
in sequence on the request.
NULL, to override the default
FUN, validated and perhaps modified.