Fix undefined index notice

Add array_key_exists check to fix undefined index notice, which was causing a headers already sent error in class-opengraph-admin.php
remove erroneous new line at end of file
Add WPSEO_Meta::get_post_value()
Update WPSEO_Social_Admin::og_data_compare() to use new WPSEO_Meta::get_post_value() method
remove dead code after refactor in WPSEO_Social_Admin::og_data_compare()
Add tests for WPSEO_Meta::get_post_value
Add missing ; in test-class-wpseo-meta.php
Merge pull request #1768 from mikeaag/trunk

Fix for https://github.com/Yoast/wordpress-seo/issues/1767