Update wh_number_manager.erl
Merge pull request #44 from freefrog/master

Fix problem of deleting/re-adding numbers -- THANKS!

For reference rather than a case clause this could be done as function clauses which we would consider a tad cleaner.  EG:
                    (#number{current_state = <<"available">>}=N) -> N;
                    (#number{}=N) -> wnm_number:error_number_exists(N)