Return a single object for single-col enumerator

Optiq expects that an enumerator of a single column returns the
single value of that column, and not an array with length 1.
The CsvEnumerator is changed to be generically typed as Object
instead of Object[], and return a single object when only a single
column is being read.
Merge pull request #1 from gabrielreid/single-column-bug

Return a single object for single-col enumerator.