Trello #42 - Fix for bug when there's a single solution

If there's a single solution associated to the account and the
assignment, the method Solution.find_by_account_and_assignment returns a
single instance of Solution instead of an array of 1 element. Updated
code to work on that scenario.
Merge pull request #10 from fiuba/fix_status_assignment_single_solution

Trello #42 - Fix for bug when there's a single solution