fix base64 decode related builtin function

[ysugimoto]

Fixes #255

Fastly terminates null-byte on a decoded string in base64 decoding functions:

• digest.base64_decode
• digest.base64url_decode
• digest.base64url_decode_nopad

Additionally, digest.base64url_decode_nopad also stops decoding when a padding sign (=) is found.
This PR follows Fastly behavior that describes the documentation.

[richardmarshall]

Proper handling of null bytes in the decode output and ignoring decoding errors definitely gets us closer to matching Fastly's behavior but there's still one issue.

Invalid characters are skipped entirely when decoding, and do not contribute to the bits for the current byte being output. Decoding continues for subsequent data as if the invalid character were not present.

Since the go decoder stops processing the input on invalid characters they should first be removed from the input before passing to the decoder.

[ysugimoto]

I looked into Fastly documentation and improved base64-ed string dealing with corresponding functions.
Adding more edge test cases and be sure Faslty's actual behavior: https://fiddle.fastly.dev/fiddle/b2e84dcd

[richardmarshall]

Overall looks good, just a couple questions on the diff.

Sorry for not getting to this sooner I briefly looked at it when you submitted it and didn't have time to do a real review and forgot to leave the notification marked as unread. Oops.

[richardmarshall]

LGTM