problems with GNFA.to_regex

GNFA.to_regex seems to give wrong results sometimes. For example, the following code:
```
from automata.fa.dfa import DFA
from automata.fa.nfa import NFA
from automata.fa.gnfa import GNFA

nfa = NFA.from_regex('a*')
dfa = DFA.from_nfa(nfa)
gnfa = GNFA.from_dfa(dfa)
print(gnfa.to_regex())
```
prints a single-character string `*`, which is not a valid regex. 
