Duplicate field error when indexing single value fields

In apachesolr_fields_default_indexing_callback(...) we take a copy of the first value of the field and store it in a singular index. This works great for multi-value fields ($field_info['multiple'] == true), but results in us trying to index the same field twice with the same name if the field is already singular.

My patch add a check for $field_info['multiple'] to prevent this and makes the code a little easier to read by moving code that's designed to execute 0 or 1 times out of the for loop.