BUG: When using a Class for grouping commands `--self` shows as a flag

### Example code (example.py):
```python
import fire

class Foo:
def bar(self):
return 'bar'

def do_stuff(self, arg1):
return 'stuff'

if __name__ == '__main__':
fire.Fire(Foo)
```

### Run:
`python3 example.py -- --completion`

### Completion Output (take note of `--self` in output):
```bash
# bash completion support for example.py
# DO NOT EDIT.
# This script is autogenerated by fire/completion.py.

_complete-examplepy()
{
local cur prev opts lastcommand
COMPREPLY=()
prev="${COMP_WORDS[COMP_CWORD-1]}"
cur="${COMP_WORDS[COMP_CWORD]}"
lastcommand=$(get_lastcommand)

opts=""
GLOBAL_OPTIONS=""

case "${lastcommand}" in

bar)

if is_prev_global; then
opts="${GLOBAL_OPTIONS}"
else
opts="--self ${GLOBAL_OPTIONS}"
fi
opts=$(filter_options $opts)
;;

do-stuff)

if is_prev_global; then
opts="${GLOBAL_OPTIONS}"
else
opts="--arg1 --self ${GLOBAL_OPTIONS}"
fi
opts=$(filter_options $opts)
;;

example.py)

opts="bar do-stuff ${GLOBAL_OPTIONS}"
opts=$(filter_options $opts)
;;
esac

COMPREPLY=( $(compgen -W "${opts}" -- ${cur}) )
return 0
}

get_lastcommand()
{
local lastcommand i

lastcommand=
for ((i=0; i < ${#COMP_WORDS[@]}; ++i)); do
if [[ ${COMP_WORDS[i]} != -* ]] && [[ -n ${COMP_WORDS[i]} ]] && [[
${COMP_WORDS[i]} != $cur ]]; then
lastcommand=${COMP_WORDS[i]}
fi
done

echo $lastcommand
}

filter_options()
{
local opts
opts=""
for opt in "$@"
do
if ! option_already_entered $opt; then
opts="$opts $opt"
fi
done

echo $opts
}

option_already_entered()
{
local opt
for opt in ${COMP_WORDS[@]:0:COMP_CWORD}
do
if [ $1 == $opt ]; then
return 0
fi
done
return 1
}

is_prev_global()
{
local opt
for opt in $GLOBAL_OPTIONS
do
if [ $opt == $prev ]; then
return 0
fi
done
return 1
}

complete -F _complete-examplepy example.py
```