Remove case sensitivity for converter column types 

SQLalchemy's columns can be DATE (notice, all CAPS), so it was giving me an error "Exception: Unsupported filter type" because it did not match the "Date" converter.

I think you can reproduce the "DATE" issue by using SQLalchemy's reflect on a table with a date column like this:
```
db.metadata.reflect(db.engine)

class MyTable(db.Model):
	__tablename__ = 'my_table'
	pass
```

Just remove the case sensitivity from Flask-Admin to prevent this issue.