Assignment Rspec Tests

Wrote tests for get_current_assignment method in Assignment model

Test cases:
* when all assignments are due in more than 3 days --> returns the assignment that is due first
* when all assignments are past the due date --> returns the assignment that was due most recently
* when there is an assignment due in 3 days --> returns the assignment due in 3 days
* when the next assignment is due in more than 3 days --> returns the assignment that was most recently due
* when no assignments are found --> returns nil
* when one assignment is found --> returns the only assignment