fixed structfield comparison when dataType is array

[dfarren]

The [newly created] test below fails. This because are_structfields_equal doesn't check for the case when the dataType is an array. If the dataType is array, then the nullability shouldn't matter when argument ignore_nullability=True.

def it_can_perform_nullability_insensitive_comparisons_with_arrays():
    sf1 = StructField("hi", ArrayType(IntegerType(), True), False)
    sf2 = StructField("hi", ArrayType(IntegerType(), False), True)
    assert are_structfields_equal(sf1, sf2, ignore_nullability=True) == True

This PR fixes this by adding check_type_equal_ignore_nullable(sf1, sf2) in the are_structfields_equal function.

[MrPowers]

Does this work for nested array columns?