Add `full_text` field back into tweet model

[praveenperera]

If you use extended_mode for getting tweets the tweet does not have a text field, instead it contains a full_text field.

ex:

ExTwitter.search("search-term", count: 20, tweet_mode: "extended")

Reference:

https://developer.twitter.com/en/docs/tweets/tweet-updates

[coveralls]

Coverage remained the same at 81.818% when pulling b1e827ebdde2567d5c546bf74ba21fd1b6bc15cd on praveenperera:patch-1 into 5de6a84c7e4529cacbf572be605fe25099b33a68 on parroty:master.

[parroty]

Thanks!