Issue #1220: data.target not set on the 'tweet' web intent event if the tweet button was created by createShareButton

For tweet buttons created with twttr.widgets.createShareButton, the tweet web intent event is called without data.target set to the iframe of the button originally pressed. data.target is properly set if the button was automatically created from a <a href="https://twitter.com/share" class="twitter-share-button">.