Fix string.len (includes new tests.)

src/core/org/luaj/lib/StringLib.java

@@ -395,7 +395,7 @@ public class StringLib extends LFunction {
 	 * Embedded zeros are counted, so "a\000bc\000" has length 5. 
 	 */
 	static void len( LuaState vm ) {
-		int l = vm.tostring(2).length();
+		int l = vm.tolstring(2).length();
 		vm.resettop();
 		vm.pushinteger( l );
 	}

src/test/res/strlib.lua

@@ -68,3 +68,12 @@ t( 'abc' )
 t( 'ab' )
 t( 'a' )
 t( '' )
+
+print(string.len("Hello, world"))
+print(#"Hello, world")
+print(string.len("\0\0\0"))
+print(#"\0\0\0")
+print(string.len("\0\1\2\3"))
+print(#"\0\1\2\3")
+local s = "\194\161Hola!"
+print(s, string.len(s), #s)