This change makes the VendorChecksumTokenGenerator check the token's checksum

when validating tokens. This makes the token validation more precise and is

useful for weeding out any false positives during secret scanning.
This also fixes the character set that we're using for base62-encoding the

token checksums. The previous one had incorrectly swapped the placement of the

capital and lowercase characters in the set. Tokens that were generated using

the old character set are still considered valid.

Ran unit tests.