Fix homogenize not working with tuples

[Zeroji]

Also fixes data corruption bug due to lists being mutable.

With agate 1.6.0, default homogenize example:

>>> table.homogenize('year', range(1997, 2003), (0, 0))
AttributeError: 'tuple' object has no attribute 'insert'
>>> table.homogenize('year', range(1997, 2003), [0, 0])
(('year', Decimal('1997')), ('f', Decimal('2')), ('m', Decimal('1')))
(('year', Decimal('2000')), ('f', Decimal('4')), ('m', Decimal('3')))
(('year', Decimal('2002')), ('f', Decimal('4')), ('m', Decimal('5')))
(('year', Decimal('2003')), ('f', Decimal('1')), ('m', Decimal('2')))
(('year', 1999), ('f', 0), ('m', 0))
(('year', 1998), ('f', 1999), ('m', 0))
(('year', 2001), ('f', 1998), ('m', 1999))

With fix (for both tuples and list arguments):

(('year', Decimal('1997')), ('f', Decimal('2')), ('m', Decimal('1')))
(('year', Decimal('2000')), ('f', Decimal('4')), ('m', Decimal('3')))
(('year', Decimal('2002')), ('f', Decimal('4')), ('m', Decimal('5')))
(('year', Decimal('2003')), ('f', Decimal('1')), ('m', Decimal('2')))
(('year', 1999), ('f', 0), ('m', 0))
(('year', 1998), ('f', 0), ('m', 0))
(('year', 2001), ('f', 0), ('m', 0))

Edit: fixes (hopefully) #699