Fix the bug in #keys matching the given pattern

It looks like here we have an inconsistent result:
```ruby
r.keys #=> ["database", "above", "suitability", "able", "ololo"]

if fakeredis?
  r.keys "ab*"
  #=> ["database", "above", "suitability", "able"]
else
  r.keys "ab*"
  #=> ["above", "able"]
end
```

The issue with previous test example seems to be located in the produced regexp /key:/. It doesn't match "akeyd" and "key1", because they don't contain a colon.