Fixes #434

Problem

tabulate() raises TypeError when sequence-valued parameters like rowalign or maxheadercolwidths are passed as tuples instead of lists:

from tabulate import tabulate

tabulate([[1, 2], [3, 4]], rowalign=("top", "bottom"))
# TypeError: can only concatenate tuple (not "list") to tuple

tabulate([[1, 2], [3, 4]], rowalign=["top", "bottom"])  # works

All other sequence-valued parameters (colalign, floatfmt, maxcolwidths, etc.) accept tuples, so this is an inconsistency.

Root cause

_expand_iterable does original + [default] * n. When original is a tuple, tuple + list raises TypeError.

Fix

Coerce original to a list before concatenation:

# before
return original + [default] * (num_desired - len(original))
# after
return list(original) + [default] * (num_desired - len(original))