RecordArray#length not dependent on the _ids property anymore

RecordArray#length based on the _ids property when available leads to
an array with null values. The length property is now based on the
content only (ArrayProxy#length).

See http://jsbin.com/aroxed/2/edit for more details.
Bring back commented lines of code.
Bring back commented lines of code.
Merge pull request #35 from karlguillotte/master

RecordArray#length not dependent on the _ids property anymore