The urlEncode function supplied by RApache has a few behaviors I don't expect.
First, it requires only character arguments, so urlEncode(1) throws
an exception. Second, it dies on the empty string.
This function is a wrapper for RApache's urlEncode that handles all
these cases. It might later be replaced with another implementation, but
the interface will stay. (The "aps" stands for AnalysisPageServer.)