Undefined is treated as zero if it ever gets to this point anyway, so no
value is added by having this default param here.
A test fixture to verify this behaviour is added.
All the `invalid2` tests have been removed as they were not invalid
base58check. They were actually valid in some cases.
They will be re-integrated in more specific bitcoin core tests in
relation to Address/ECKey respectively.
After a long IRC discussion, it was decided that the use of direct
filepaths instead of the module is a more pure form of testing ,
although it may provide less overall coverage than the mixed integration
style imports used previously.
This will need to be remedied by further integration testing in
/test/integration.
