allow Script#to_pubkey_script_sig without pubkey (for pay-to-pubkey and pay-to-pubkey-hash)
allow Script#to_pubkey_script_sig without pubkey (for pay-to-pubkey and pay-to-pubkey-hash)

  todo: maybe rename it because its name is wrong and confusing

mhanne/bitcoin-ruby,assafshomer/bitcoin-ruby,georgemillo/bitcoin-ruby,robertg/bitcoin-ruby,xiumiao/bitcoin-ruby,tarix/bitcoin-ruby,Zlerp/bitcoin-ruby,bitpesa/bitcoin-ruby