.set() should not overwrite existing deferreds; instead,
it should resolve the existing deferred if it's pending.
This is necessary because .set() is used by processResult().
Without this, passing .get() a title that no information
is known for results in a promise that is never resolved,
because the associated deferred is overwritten as soon
as the API response arrives.
Still make .set() a no-op if data has already been set,
by checking if the deferred is pending. For .resolve() this
doesn't matter, but for modifying this.cacheValues it does.
Bug: T107212
Change-Id: I70e8c5450f23062db214ccc5c585624d41de6509
Per Timo, this avoids us having pointless code that is never
executed, and the replacement notification to the user (that
ve.blah.UnicornAnnotation.foo is null and not a method) is
sufficiently clear that the issue lies in the lack of existence
of the ve.blah.UnicornAnnotation#foo method.
Only ones in this repo appear to be in ve.init.mw.ApiResponseCache.
Bug: T54482
Change-Id: If44b8d63b8e7b2a5c289fe7278ad54e722b185fa
