use subjects for generating topic name instead of name from streamconfig

[grumblebrian]

Motivation / Background

The JetStream Publish function uses streamConfig.Name for the topic when marshaling the message, when it should be using streamConfig.Subjects. This is not a problem when using defaultStreamConfigurator since that sets the passed in topic as the name while also setting the first Subjects item to the topic. However, when using a custom (existing) stream config it is likely that the name will be different from the topic. This is because topics typically contain periods to reflect a topic hierarchy and periods cannot be used in stream names. Thus, a custom jetstream.StreamConfig likely appears similar to the following:

jetstream.StreamConfig{
    Name: "COMMAND_TEST",
    Subjects: []string{"command.test"},
    Storage: jetstream.FileStorage,
    Retention: 72 * time.Hour,
}

Typically, stream names are capitalized but that is not required.

Details

This was a simple change to use streamConfig.Subjects[0] instead of streamConfig.Name for the topic when calling the Marshal function. This does not change the default behavior when the ConfigureStream PublisherConfig option is not used when creating a JetStream publisher since defaultStreamConfigurator sets both Name and Subjects with the passed-in topic.

Checklist

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• I wrote tests for the changes.
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[christian-gama]

This is indeed important. I often have to manually setup the streams due to this watermill behavior.