Gplus meta bugfix

If opengraph meta data were enabled and no fb title was written, SEO plugin filled og:title with post title, same logic for og:description and og:image.
Google plus meta tags didn't work like that, they simply were not displayed if not specified.
I think a refactor of class WPSEO_GooglePlus should be planned to enhance consinstency with WPSEO_OpenGraph and WPSEO_Twitter.
Meanwhile itemprop=name, itemprop=description and itemprop=image work like opengraph ones for post view.