<fix(datepicker)>: change mdDateUtil.isDateWithinRange to ignore the …

[ademuri]

…time component of the date

Previously, mdDateUtil.isDateWithinRange did a simple comparison on the given dates. Since this
component is used for dates (and not times or datetimes), this leads to inconsistent behavior.

Fixes #6887

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CLAs look good, thanks!

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CLAs look good, thanks!

[ThomasBurleson]

@jelbourn - can you review ?

[jelbourn]

Instead of angular.isDate, use isValidDate

[ademuri]

Done.

[jelbourn]

LGTM aside from one comment. Great contribution, thanks!

[ademuri]

PTAL