fix get_unspent_txouts() for multisig with same addresses but different ...

...signatures required
spent some time on this and I think I understand why you think that, but :

- `decode_checkmultisig` and `decode_checksig` don't return address for output with data, and here we need this outputs to eventually spent them.
- operations done by `scriptpubkey_to_address()` are really more basic than these two functions, and very specific to this context. I get only `signatures_required` and `signatures_possible` from the script, and the addresses from they array of addresses provided in the json returned by bitcoind. It's only an aesthetic function to have a clean `if`.

I just add two commits with:
- use of util.construct_array() to build the address
- make `scriptpubkey_to_address()` an internal function of `get_unspent_txouts()`

tell me if look better now.