Bugfix: Don't screw up function calls in index definitions

There was a bug introduced in commit 06d8950bd7 which causes index definitions like this to produce invalid SQL:
```
has 'COALESCE(`parties`.`bankrupt_from`, `parties`.`inoperational_from`, `parties`.`terminated_from`, `parties`.`liquidation_from`)', :as=>:nonfunctional, :type => :timestamp
```

The bug caused TS to split on every comma and wrap each part individually with UNIX_TIMESTAMP().
Commit 07132f2 fixes that. You already have the previous commits in your dev branch.