Add specular/specularMap passthrough.

examples/js/loaders/GLTFLoader.js

@@ -1100,8 +1100,14 @@ THREE.GLTFLoader = ( function () {
 
 					materialParams.specular = new THREE.Color().fromArray( materialValues.specular );
 
+				} else if ( typeof( materialValues.specular ) === 'string' ) {
+
+					materialParams.specularMap = dependencies.textures[ materialValues.specular ];
+
 				}
 
+				delete materialParams.specular;
+
 				if ( materialValues.shininess !== undefined ) {
 
 					materialParams.shininess = materialValues.shininess;