Fixing abort errors

I have an error https://github.com/jasonlong/geo_pattern/blob/master/lib/geo_pattern/pattern.rb#L50-L55 is aborting because there is a generator in the defaults that is `''`. https://github.com/jasonlong/geo_pattern/blob/master/lib/geo_pattern/pattern.rb#L7-L10

Removing the default `:generator => ''` will skip the block as intended when no generator is included.

@jasonlong @andrew