Reset the data in mi to the initial state

[sturmianseq]

The PR aims to improve the reliability of the test test_insert_overwrite by resetting the data in object mi to the initial state by calling insert to insert the original tuple.

The test can fail in the following way if im is not in the initial state:

>       assert emp_seq[0] == mi.get('first_name', 'Steve')
E       AssertionError: assert <multiindex.tests.test_multiindex.Employee object at 0x7f48a4106f40> == <multiindex.tests.test_multiindex.Employee object at 0x7f48a411a100>
E        +  where <multiindex.tests.test_multiindex.Employee object at 0x7f48a411a100> = <bound method MultiIndexContainer.get of <multiindex.multiindex.MultiIndexContainer object at 0x7f48a411ae80>>('first_name', 'Steve')
E        +    where <bound method MultiIndexContainer.get of <multiindex.multiindex.MultiIndexContainer object at 0x7f48a411ae80>> = <multiindex.multiindex.MultiIndexContainer object at 0x7f48a411ae80>.get