Catch StandardError during Base64 decoding in message encryptor.

[simi]

Motivation / Background

RubyGems.org got recently facing 500 HTTP responses since user mangled cookies (app uses cookie store) and provided custom value which (thanks to series of edge-cases) ended up triggering NoMethodError: undefined method unpack1' for nil raised down from Base64 class. Per my understanding there is no way to catch this in application and this needs to be fixed in Rails itself.

Detail

I have tracked this issue down to ActiveSupport::Messages::Code class expecting Base64 failing with ArgumentError only. But Base64 can fail also with other exceptions when wrong input type provided. In the shared case nil is actually passed to Base64.strict_decode64 resulting into NoMethodError. I have changed rescue to handle StandardError since similar approach is used few lines bellow in deserialize method.

As an side effect, this tests this unhappy branch of decode method which wasn't covered by active support test suite before.

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I have originally considered updating Base64 to raise ArgumentError when nil passed, but there are other values which can still raise similar error and it would be needed to check if passed object responds to methods needed for Base64 calculation and that's not usually happening in stdlibs.