Fix bug on sorting select fields.

The sort option for select fields is not working due to a bad binding on
the function provided to sortBy. This commit fixes the binding so the field
options can be accessed from that function.
Default select sorting to the displayed attribute.

Select fields would always be sorted, normally by the text it displays, so
the user can find the option easier.
Sorting attribute can still being changed with the sortOptionsBy.
Merge pull request #12 from grilix/master

Fix a bug on select_field sorting.