Office mime types update fix

To be able to debug different migration situations, I have moved the office mime type code to a repair step.

The second test case will fail for now because the current code doesn't work when the target mime type already exists.

This is a possible fix for https://github.com/owncloud/core/pull/8895

- [ ] add precondition to prevent re-running the same repair steps needlessly
- [x] fix the "UPDATE" SQL statement for the case where the target value already exists
- [x] investigate mime type discrepancies on master (see https://github.com/owncloud/core/pull/8895#issuecomment-47134711)

@butonic @icewind1991 

Fixes #9232
Fixes #9196