Hi,

I've two more issues where format behavior should probably be identical:

1: (version 2.6 vs. 3.1):

Version 2.6:
>>> format(Decimal("NaN"), "+08.4")
'+0000NaN'
>>> format(float("NaN"), "+08.4")
'+00.0nan'

Version 3.1:
>>> format(Decimal("NaN"), "+08.4")
'+NaN    '
>>> format(float("NaN"), "+08.4")
'+0000nan'


2: (float vs. decimal):

>>> format(float(123), "00")
'123.0'
>>> format(Decimal(123), "00")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.1/decimal.py", line 3611, in __format__
    spec = _parse_format_specifier(specifier, _localeconv=_localeconv)
  File "/usr/lib/python3.1/decimal.py", line 5563, in
_parse_format_specifier
    raise ValueError("Invalid format specifier: " + format_spec)
ValueError: Invalid format specifier: 00