Update specs to make Rubinius compliant with masgn_spec return value

Previously, Rubinius was explicitly non-compliant with regards to the
return values of mass-assignment expressions, always returning true
instead.
Update compiler specs to return arrays of values from mass assignments
Return the correct value from mass-assignment expressions

Fixes #1765
Merge pull request #2322 from leocassarani/masgn

Return the correct value from mass-assignment expressions