Returns the receiving buffer with the string new, padded or truncated to length length, inserted into the MutableBuffer after the n'th character. The default value for n is 0, which means insertion at the beginning of the string. If specified, n and length must be positive integers or zeros. If n is greater than the length of the buffer contents, the string new is padded at the beginning. The default value for length is the length of new. If length is less than the length of string new, insert truncates new to length length. The default pad character is a blank.