Moved the code that converts a gallery span to the gallery shortcode when saving an uploaded post.

The code had no effect because it was applying to `postContentEditable` after it had already been applied to the `content` string.

Note: Galleries still saved fine for local drafts because that is handled in the WPHtml.toHtml() method.

Fixes #1437
Merge pull request #1438 from wordpress-mobile/issue/1437-gallery-uploaded-post

Fix #1437: Gallery shortcodes not saving when editing uploaded post