search_next_results does not pass on certain opts

[foggy1]

Long story short, the Twitter API has this documented-in-one-place thing called tweet_mode where you get the full_text instead of the text. If you pass metadata from a search to a search_next_results, it does not pass on the tweet_mode=extended param. Even if I fudge it and manually add the &tweet_mode=extended to next_results on the metadata before passing it in, all tweets fetched by search_next_results in this fashion will only return text and not full_text.

Since you can pass max_id from next_results as an opt to plain search in itself to get the next page of results, it seems prudent to perhaps rely on this and encourage the manual merging of opts, rather than using next_results which is incomplete. (Alternatively, it can also just auto-merge the max_id into the existing opts as a default).

If this isn't objectionable (I might be missing something about next_results), then I'd be very happy to PR this. Let me know!