Purging of cached output from Special:EntityData.

This will cause Special:EntityData to purge HTTP caches
if action=purge is given in the URL. Note that currently,
only the presently requested format of the output is purged.

Change-Id: Ic13fb09ad5abbeafe2b527c07245cf20ea4f737a
Merge "Purging of cached output from Special:EntityData."