Date parser doesn't properly parse ISO date

[nijel]

The simple query datefield:2019-02-02 seems to be parsed as datefied:2019 by the date parser.

I was able to make it parse properly by giving higher priority to simple parser here:

whoosh/src/whoosh/qparser/dateparse.py

Lines 718 to 719 in 26153e2

	self.bundle = Choice((self.plusdate, self.datetime, self.simple),
	name="bundle")

The new order reads: (self.plusdate, self.simple, self.datetime)

But I'm not really sure if it won't have some bad side effects. For my particular purpose parsing ISO date is more important.