Fixed Unicode corruption bug

When multibyte characters and single byte characters are mixed, `QR8bitByte` result in corrupted byte sequence.
For example, `QR8bitByte("あa")` is expected to result in `[0xE3, 0x81, 0x82, 0x61]` but actually `[0xE3, 0x81, 0x82, 0x61, 0x81, 0x82]`. This is due to reuse of buffer array.
This patch creates a new array for each iterations, and then concat them in bulk.