Raise InvalidRequest if the query is not a string

[sbscully]

• Currently if an invalid location/query is passed to the OpenCage::Geocoder#geocode method it will still make a request to the OpenCage API and it might not be immediately clear that the arguments were invalid.
• Throws an OpenCage::Error::InvalidRequest if anything other than a string is passed for the query/location.

[mtmail]

I had to double-check what happens if the query is nil because that's not a String type either. The library already raised the same error so the change is backward compatible.