Bare asterisk fix breaks docstrings

[Denis Zawada]

It works but breaks docstrings:

#!/usr/bin/env python3

def func(obligatory, *, other=' ', arguments=''):
    """func(obligatory, [other, arguments]) -> ponies

    Returns ponies for the given `obligatory` argument. Optional `other`
    `arguments` may be passed."""

    return "{} ponies.{}{}".format(obligatory, other, arguments)

print(func.__doc__)
print()
print(func("Pink", arguments="Not yours."))

is converted to

#!/usr/bin/env python

def func(obligatory, **_3to2kwargs):
    if 'arguments' in _3to2kwargs: arguments = _3to2kwargs['arguments']; del _3to2kwargs['arguments']
    else: arguments = u''
    if 'other' in _3to2kwargs: other = _3to2kwargs['other']; del _3to2kwargs['other']
    else: other = u' '
    u"""func(obligatory, [other, arguments]) -> ponies

    Returns ponies for the given `obligatory` argument. Optional `other`
    `arguments` may be passed."""

    return u"{} ponies.{}{}".format(obligatory, other, arguments)

print func.__doc__
print
print func(u"Pink", arguments=u"Not yours.")

Now func.__doc__ is None.

(Inherited from https://bitbucket.org/amentajo/lib3to2/issue/21)