Eliminate copying of Huffman encoded string in QpackInstructionEncoder. Eliminate copying of Huffman encoded string by moving HuffmanEncode() call from QpackInstructionEncoder::DoWriteString() to QpackInstructionEncoder::DoStartString(). No functional change. PiperOrigin-RevId: 336098147 Change-Id: I0d7322b0a2dcd2b95a1a0bc5d4aea0b73b1edbd8