Exact sematic of base64-decode-string

[Igor Bukanov]

I see that base64-decode-string is quite strict about its input. In
particular, it rejects strings without = padding or with non-base64
characters. On the other hand it accepts and decodes without any error
an invalid string like "QR==":

(base64-decode-string "QR==") gives "A" when the correct base64
encoding for "A" is "QQ==".

Is it intentional or simply a bug in the implementation?