Fix QoS 2 message rejections to send PUBREC instead of PUBACK, allow errRejectPacket through

[SimonMacIntyre]

Fix QoS 2 message rejections to send PUBREC instead of PUBACK and remove the errRejectPacket no-op check.

I am unsure why the line which I removed was there, but that was stopping that packet from falling through to the lower if block where it would have made a puback as needed for qos1. However, for qos2 it should be pubrec so I addressed that here too.

Link to spec

Fixes #465

[coveralls]

Pull Request Test Coverage Report for Build 15562075462

Details

• 16 of 20 (80.0%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.02%) to 98.553%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
server.go	16	20	80.0%

Files with Coverage Reduction	New Missed Lines	%
server.go	1	98.97%

Totals
Change from base Build 13605453911:	0.02%
Covered Lines:	6200
Relevant Lines:	6291

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💛 - Coveralls