Update get_attachment_link() to return image URL when attachment pages are turned off.

[deepakrohillas]

Trac ticket: https://core.trac.wordpress.org/ticket/61133

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Props deepakrohilla, peterwilsoncc.

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[peterwilsoncc]

I think the changes you've made can be earlier in the function and bypass the check for a parent.

I'd suggest placing the new condition immediately after the call the get_post():

	$post = get_post( $post );
	// Return image URL if attachment pages are disabled.
	if ( $post && '0' === get_option( 'wp_attachment_pages_enabled' ) ) {
		return wp_get_attachment_url( $post->ID );
	}

This change will probably cause a few unit tests to fail as the value of get_attachment_link() will no longer meet the expected value. These tests should be updated to check the correct value is returned if the attachment pages are enabled, and if they are not.

I'd suggest duplicating the existing tests, once to run with pages enabled, and one to run with them disabled.

[deepakrohillas]

@peterwilsoncc : Thanks for the review. Suggested changes has been done.