JSON Key Path Fails to Load Even When Readable

[inspiredstuffs]

About this line: https://github.com/decision-labs/fcm/blob/ec489fa22dd11185d487f781c009bca782144d17/lib/fcm.rb#L296C22-L296C35

I suggest the json_key method should look like this:

 def json_key
    @json_key ||= if @json_key_path.respond_to?(:read)
                    @json_key_path.read
                  else
                    File.open(@json_key_path)
                  end
  end

This will make it compatible with an instance of StringIO object etc.