Filter headers

[filosofo]

Problem: Currently there's no good way to control headers via a plugin. For example, under the status quo if someone is logged in the headers are set not to cache, which may not be desirable for a high-traffic community site. Also, others have complained about not being able to remove certain headers.

Solution: Filter the headers printed by the send_headers method of WP.

Patch also adds a wp_get_nocache_headers() function to filter the headers used in nocache_headers().

(In [10619]) Allow for the HTTP headers returned by WordPress to be filtered by a plugin. Fixes #9205 props filosofo.

This causes a php error for the dead_db message ATM as it uses nocache_headers when apply_filters isn't yet available.

(In [10626]) Wrap the apply_filters call in a function_exists check as this can be called before it is defined. See #9205.