[bug] short flags/arguments does not work with `**kwargs`

运行 issue 中的两个 Python 示例，并比较有无 kwargs 时短选项的行为。然后跟踪 Python Fire 对短选项和关键字参数的参数解析过程。完成标准是：-f 和 -s 填充 first_arg 和 second_arg，同时无关的 flag 仍出现在 kwargs 中，并且两个情况都有回归测试覆盖。

Description:

Short flags/arguments (e.g. -u for --username) does not work with **kwargs

Example that works OK:

The following example works ok:

import fire

def cli(first_arg="left", second_arg="right"):
    """Prints the 2 arguments separated by bar
       
    Parameters
    ----------
    first_arg : str
        the first arg
    second_arg : str
        the second arg
    """
    print(f"{first_arg} | {second_arg}" )

if __name__ == '__main__':
  fire.Fire(cli)

Help output

❯ python .\example.py -- --help
NAME
    example.py - Prints the 2 arguments separated by bar

SYNOPSIS
    example.py <flags>

DESCRIPTION
    Prints the 2 arguments separated by bar

FLAGS
    -f, --first_arg=FIRST_ARG
        Default: 'left'
        the first arg
    -s, --second_arg=SECOND_ARG
        Default: 'right'
        the second arg

Usage with short flags

❯ python .\example.py -f="hello" -s="word"
hello | word

Example that does not work:

The following next example does not work with short flags, even when they are showing on the help output.
It is using **kwargs. It seems the short flags are going to the **kwargs dictionary.

import fire

def cli(first_arg="left", second_arg="right", **kwargs):
    """Prints the 2 arguments separated by bar
       
    Parameters
    ----------
    first_arg : str
        the first arg
    second_arg : str
        the second arg
    kwargs : str
        additional args
    """
    print(f"{first_arg} | {second_arg}" )
    print(kwargs)

if __name__ == '__main__':
  fire.Fire(cli)

Help output

❯ python .\example2.py -- --help
NAME
    example2.py - Prints the 2 arguments separated by bar

SYNOPSIS
    example2.py <flags>

DESCRIPTION
    Prints the 2 arguments separated by bar

FLAGS
    -f, --first_arg=FIRST_ARG
        Default: 'left'
        the first arg
    -s, --second_arg=SECOND_ARG
        Default: 'right'
        the second arg
    Additional flags are accepted.
        additional args

Usage with short flags

❯ python .\example2.py -f="hello" -s="word"
left | right
{'f': 'hello', 's': 'word'}

In summary: the -f and -s arguments should be parsing to --first_arg and --second_arg but they are parsing to kwargs