(bug 43251) API prop=pageprops ppprop should accept multiple values

There is no reason for this parameter not to accept multiple values, the
query is structurally exactly the same as for something like
prop=categories&clcategories=....

Unless, of course, we need to support page props with "|" in the name.

Change-Id: I38cd83e213ede8e626d55c079c5c1b6f7e727336