(PUP-2291) Fix absent priorities in yumrepo

The recent changes in puppet 3.5.0 have broken the ability to set the
priority of a yumrepo as absent.

This commit fixes that.

yumrepo{
    'foo':
        priority => absent,
}
(pup-2292) stronger tests on yumrepo's properties

When you set => absent on a yumrepo property, the result is a string in
ruby. However, it happens that we expect only a :symbol there in the
tests.
Merge commit 'refs/pull/2543/head' of git://github.com/puppetlabs/puppet into merge/yumrepo-priority-absent
Merge commit 'refs/pull/2544/head' of git://github.com/puppetlabs/puppet into merge/yumrepo-priority-absent
Merge branch 'merge/yumrepo-priority-absent'

This closes GH-2543, GH-2544, and GH-2545.