Fix creating a wallet after changing Bitcoin.chain_params fails. (#31)

[JunOkumura]

This PR solve #31.

I run below, it is complete.

$ ./bin/console 
irb(main):001:0> wallet = Bitcoin::Wallet::Base.create(99999)
=> #<Bitcoin::Wallet::Base:0x00007fc72d101ef8 @path="/Users/okumura/.bitcoinrb/mainnet/db/wallet/wallet99999/", @db=#<Bitcoin::Wallet::DB:0x00007fc72c9f1dc0 @level_db=<LevelDB::DB "/Users/okumura/.bitcoinrb/mainnet/db/wallet/wallet99999/">, @master_key=#<Bitcoin::Wallet::MasterKey:0x00007fc72d0b4860 @mnemonic=["piece", "return", "maid", "learn", "dish", "stereo", "snow", "melt", "bounce", "lottery", "shock", "empty", "miss", "govern", "carpet", "tail", "turn", "often", "junior", "monster", "tell", "warrior", "grab", "candy"], @seed="67c0098389f99da10a5d15f7b65b9f095d60f77c6a1bff4a0b7185c6f09482872dab3ee80235791cccc5eab540a12b2e52bec0ba81b551c35a30d3114f7fe983", @encrypted=false, @salt="">>, @wallet_id=99999>
irb(main):002:0> wallet.close
=> true
irb(main):003:0> Bitcoin.chain_params = :regtest
=> :regtest
irb(main):004:0> wallet = Bitcoin::Wallet::Base.create(99999)
=> #<Bitcoin::Wallet::Base:0x0000000105731b20 @path="/Users/okumura/.bitcoinrb/regtest/db/wallet/wallet99999/", @db=#<Bitcoin::Wallet::DB:0x0000000105731aa8 @level_db=<LevelDB::DB "/Users/okumura/.bitcoinrb/regtest/db/wallet/wallet99999/">, @master_key=#<Bitcoin::Wallet::MasterKey:0x00000001056ba458 @mnemonic=["iron", "tower", "speak", "identify", "enlist", "crucial", "bracket", "multiply", "confirm", "about", "width", "behind", "melody", "output", "judge", "payment", "doctor", "come", "capable", "remove", "liquid", "claw", "invest", "electric"], @seed="d9c308ff5bc44cec4499e71467cb70f215fd8b57e08d7ed44fdd6369df5a1945992000ef51513401360f65b068db7a37869637f3984ab172da5bfc41c4e6ed73", @encrypted=false, @salt="">>, @wallet_id=99999>
irb(main):005:0> wallet.close
=> true