#3196: if needed pad a short base64 encoded word before trying to decode.

#3196: if needed pad a short base64 encoded word before trying to decode.

The RFCs encourage following Postel's law: be liberal in what you accept.
So if someone forgot to pad the base64 encoded word payload to an
even four bytes, we add the padding before handing it to base64mime.decode.
Previously, missing padding resulted in a HeaderParseError.

Patch by Jason Williams.