Default message priority should be 4, not 0

[mkuratczyk]

According to the specification, the default priority should be 4:
https://docs.oasis-open.org/amqp/core/v1.0/os/amqp-core-messaging-v1.0-os.html#type-header
It's also mentioned in a comment:
https://github.com/Azure/go-amqp/blob/main/message.go#L306

However, currently, it is implicitly set to 0, since that's the default for uint8. For example, I believe this test should pass after removing Priority: 4 but it fails.

Similar issue in AMQP.Net Lite from a few years ago: Azure/amqpnetlite#301

[jhendrixMSFT]

Sorry for the delay.

Unfortunately, the zero-value for Priority is ambiguous. So, when it's 0 it's interpreted verbatim.

Ideally, this field would be pointer-to-type so when it's nil that means 4. That would be a breaking change at this point though.

I see two options.

• Add a NewMessageHeader() constructor that creates a MessageHeader with its Priority set to 4.
• Document the behavior, suggesting that callers set Priority to 4 when instantiating a MessageHeader instance.