Fixed StringBuilder.insert

Test code:
		StringBuilder b = new StringBuilder("test123");
		int offset = 2;
		char[] str = "ABCD".toCharArray();
		b.insert(offset, str, 1, 2);
		System.out.println("b: " + b);
		System.out.println("str: " + new String(str));

		// wrong:
		// b: tesest1
		// str: ABBC

		// right:
		// b: teBCst123
		// str: ABCD