Use "get" and "get_multiple" for Media and CustomPostType

`WP_JSON_Media` and `WP_JSON_CustomPostType` extend `WP_JSON_Posts` which no longer has `get_posts` or `get_post` defined. This fix renames the `WP_JSON_Media` and `WP_JSON_CustomPostType` method names to match the parent class as well as resolves the fatals caused by calling non-existent methods.