Calculate the hex form of a block's header, given its getblock information from bitcoind.

def block_header_to_hex( block_data, prev_hash ):
    """
    Calculate the hex form of a block's header, given its getblock information from bitcoind.
    """
    header_info = {
       "version": block_data['version'],
       "prevhash": prev_hash,
       "merkle_root": block_data['merkleroot'],
       "timestamp": block_data['time'],
       "bits": int(block_data['bits'], 16),
       "nonce": block_data['nonce'],
       "hash": block_data['hash']
    }

    return block_header_serialize(header_info)