EZP-21961: Improve coverage in FieldType

Adds coverage for `getName()` and `getFieldTypeIdentifier()` for all field types except XmlText.

Also fixes http://jira.ez.no/browse/EZP-21968 and http://jira.ez.no/browse/EZP-21967 that were discovered while writing tests.

## Todo
- [x] Extract fixes to code and create issues + PR