api.py: buffer 'pageprops' in QueryGenerator
api.py: buffer 'pageprops' in QueryGenerator

Currently, 'pageprops' is only cached if found in the response.
The API only returns this property if it's not empty, causing
repetitive queries if accessed multiple times.
This change caches 'pageprops' if it's requested, creating an
empty dictionary to use if it's not present in the response.

Change-Id: I30973b7479822ebc043f1d4db77522143e8c5652

wikimedia/pywikibot-core,wikimedia/pywikibot-core