Allow method calls for keys ending with question-mark

- Currently, if a key is created with a name like :question?,
settings.question? assumes it's checking if it exists instead
of asking for the value.
- Instead, we should first return a value if it exists with this key,
if not call settings#has?(key)
Handle nil values with keys ending in question-marks
Merge pull request #1 from nburkley/question_mark_keys

Allow method calls for keys ending with question-mark