Twenty Fourteen: simplify header image check, props obenland. See #25400.

git-svn-id: https://develop.svn.wordpress.org/trunk@25604 602fd350-edb4-49c9-b593-d223f7449a82
When is_main_query() is called during the pre_get_posts hook, issue _doing_it_wrong().

The is_main_query() WP_Query method should be used in these instances instead.
see #23329.


git-svn-id: https://develop.svn.wordpress.org/trunk@25605 602fd350-edb4-49c9-b593-d223f7449a82