[qmf] Fields which are matched case-insensitive should sort similarly

Text fields that are matched with case-insensitive matching should
also be sorted with case-insensitive collation.

Also includes fix to extend case insensitivity to matching for mail
thread keys.
Merge pull request #18 from matthewvogt/case-insensitive-sort

[qmf] Fields which are matched case-insensitive should sort similarly