CRM-19222 Data too long on export including custom field

When doing an export that includes an alphanumeric multiple-choice custom field that has a database field length, the temp table for the export will set the column length to that field length value.

In the case of a field that has very short value length but reasonably long labels, you can have a "data too long" database error when the export loads values into the temp table. An export should have the label in the field, not the value, and even though the database field length might limit the values, the labels can be up to 255 characters as the label field in civicrm_option_value is varchar(255).