Fix: enable autoshare meta always set to 0 when saving draft

[dinhtungdu]

Description of the Change

In block editor and with Yoast activated, there is an additional call that triggers save_post action, which then triggers TenUp\AutoshareForTwitter\Core\Post_Meta\save_tweet_meta. This time the REST_REQUEST constant isn't defined, which leads to the post meta autoshare_autoshare_for_twitter is always set to 0. This PR checks for the 'Enable by default' setting and use that value to update the post meta.

Fixes #102

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

[jeffpaul]

@dinhtungdu FYI, looks like there are failing tests in PHP 7.1

[jeffpaul]

Otherwise confirmed internally that this resolves the reported issue, so we'll be good to merge once we can get the tests passing.

[dinhtungdu]

@jeffpaul Tests are updated and passed now