fix 2-space rule when SPACES_BEFORE_COMMENT is a list

[YuKirasawa]

As readme says with spaces_before_comment="15, 20",

    1 + 1 # Adding values
    two + two # More adding

    longer_statement # This is a longer statement
    short # This is a shorter statement

    a_very_long_statement_that_extends_beyond_the_final_column # Comment
    short # This is a shorter statement

will be formatted as:

    1 + 1          # Adding values <-- end of line comments in block aligned to col 15
    two + two      # More adding

    longer_statement    # This is a longer statement <-- end of line comments in block aligned to col 20
    short               # This is a shorter statement

    a_very_long_statement_that_extends_beyond_the_final_column  # Comment <-- the end of line comments are aligned based on the line length
    short                                                       # This is a shorter statement

But now the behavior of yapf is formatting it as

    1 + 1          # Adding values <-- end of line comments in block aligned to col 15
    two + two      # More adding

    longer_statement    # This is a longer statement <-- end of line comments in block aligned to col 20
    short               # This is a shorter statement

    a_very_long_statement_that_extends_beyond_the_final_column # Comment <-- the end of line comments are aligned based on the line length
    short                                                      # This is a shorter statement

The last comment block is preceded by only one space. This condition is also mentioned in #675
I think the bug is caused by the below code in yapf/yapflib/reformatter.py#336

        # Calculate the aligned column value
        max_line_length += 2

        aligned_col = None
        for potential_col in tok.spaces_required_before:
          if potential_col > max_line_length:
            aligned_col = potential_col
            break

        if aligned_col is None:
          aligned_col = max_line_length

When a value is selected in the spaces_before_comment list, aligned_col will be exactly larger than max_line_length. But when no values are selected, aligned_col will be not larger than max_line_length. So I modified the later behavior to make them consistent.