Force URLs in srcset attributes into HTTPS scheme. A.K.A Fix for WP 4.4 responsive images.

functions.php

	/*
	 * Force URLs in srcset attributes into HTTPS scheme.
	 */
	function ssl_srcset( $sources ) {
	  foreach ( $sources as &$src ) {
	    $src['url'] = set_url_scheme( $src['url'], 'https' );
	  }

	  return $sources;
	}
	add_filter( 'wp_calculate_image_srcset', 'ssl_srcset' );