Cleanup after [28671]:
* Set better defaults in `paginate_links()`, so that themes don't have to calculate them on their own, like Twenty Fourteen does now.
* Don't set page 1 to `?page=1` or `/page/1/` - that will force a canonical redirect.
* Add and cleanup unit tests

Props obenland, SergeyBiryukov, wonderboymusic.
Fixes #24606.

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git-svn-id: http://core.svn.wordpress.org/trunk@28598 1a063a9b-81f0-0310-95a4-ce76da25c4cd