Do not delete :now from the options hash.

Sometimes you want to share the options Hash between multiple calls to Chronic. The existed behavior produced very confusing bugs caused by a parameter disappearing from that shared config.

About the fix. Deleting `:now` option was needed, because the provided options hash would previously be frozen and assigned to a class variable. See [1130ba2f818238aad9df2e945b0eb0285717e3c8](https://github.com/mojombo/chronic/blob/1130ba2f818238aad9df2e945b0eb0285717e3c8/lib/chronic.rb#L211). Since it’s not the case anymore, I can see no sense in mutating the function arguments.