User model test now green.

Note that fast_count method is no more.

--HG--
extra : amend_source : 489c53fa8cd1f554ab37e620cf5b7b146444c3d8
Fix and modernize broken HistoryChange#next.
Modernise HistoryChange#latest.
Remove redundant return.
Modernise History#set_previous.
Fix test for UserRecord.
Fix test for UserMailer.
Move mailer test to where it belongs.

--HG--
rename : test/unit/user_mailer_test.rb => test/mailers/user_mailer_test.rb
Only overwrite created_at if it's nil.
Fix test for Tag.
Refactor.

Mainly to avoid passing deprecated parameters to `.order`.
Use `.exists?` to check existence.
Fix test for TagSubscription.

- removed test for nonexistent method
Use find_each for batch processing.
Fix test for Tagimplication.
Modernise queries in TagImplication.
Remove x64_mingw because older bundle doesn't recognize it.