Added support for default wallet functions in multiwallet environment

[wleev]

As is the library can not access the default wallet when multiple wallets are loaded. It keeps using the URL format: host:port/ as RPC endpoint when it should be using host:port/wallet.

This solution adds a parameter so that the host:port/wallet URL will always be used, since this will work regardless of whether multiple wallets are loaded or not.

The expected failing test showcases this, but you can also verify this with bitcoin-cli like so ( assuming you start from a state where not other wallets are loaded ):
./bitcoin-cli -rpcuser=testuser -rpcpassword=testpassword -regtest getbalance
./bitcoin-cli -rpcuser=testuser -rpcpassword=testpassword -regtest loadwallet anotherWallet
./bitcoin-cli -rpcuser=testuser -rpcpassword=testpassword -regtest getbalance

After loading a new wallet the getbalance call will fail and should be changed to:
./bitcoin-cli -rpcwallet -rpcuser=testuser -rpcpassword=testpassword -regtest getbalance
Which is the same as adding /wallet to the RPC endpoint.

[pedrobranco]

Thanks for the fix. I've replaced the variable with a more explicit name allowDefaultWallet and fixed the approach. Please amend the commit @wleev.

[0x107D]

Hi guys
it's a breaking change for those who don't have build btc core with wallet support