Default the_post_thumbnail() and get_the_post_thumbnail() to return the first image in post content if no featured image is set

gistfile1.php

//If no post thumbnail exists, get the first image in the post and return that instead
add_filter( 'post_thumbnail_html', function( $html, $post_id, $post_thumbnail_id, $size, $attr  ) {

    if ( $html )
		return $html;

	$found = preg_match( '/<img.+?class=\".+?([0-9]+).*\/>/', get_post( $post_id )->post_content, $matches );

	if ( $found )
		$html = wp_get_attachment_image( end( $matches ), $size, false, $attr );

	return $html;

}, 10, 5 );