Received Message Attribute via XML is not decoded from base 64

[kojisaikiAtSony]

Problem: JSON requests has decoded from base 64 on JSON marshaller, but for XML, the SetAttributesFromForm does not have decoding step. So internal BinaryValue was still base 64 string (as binary) value.

This is a part of XML protocol, but today the actual AWS SNS and the SDK for SNS is still using XML whereas SQS has already moved to SQS (https://github.com/Admiral-Piett/goaws/issues/337). We have acknowledged that by our investigation and lack of JSON description on official SNS doc. This blocks tests that use SNS apis.

Fix: Add base 64 decoding step in SetAttributesFromForm.