Reads body part content chunk of the specified size.

        size: chunk size

async def read_chunk(self, size: int=chunk_size) -> bytes:
        """Reads body part content chunk of the specified size.

        size: chunk size
        """
        if self._at_eof:
            return b''
        if self._length:
            chunk = await self._read_chunk_from_length(size)
        else:
            chunk = await self._read_chunk_from_stream(size)

        self._read_bytes += len(chunk)
        if self._read_bytes == self._length:
            self._at_eof = True
        if self._at_eof:
            newline = await self._content.readline()
            assert newline == self._newline, \
                'reader did not read all the data or it is malformed'
        return chunk