Fix ack failure for payload with header

[vearutop]

Fixes #138.

retract v5.1.0 considered for go.mod, but it would fail build for go1.15 and below.

[github-actions]

Lines Of Code

Language	Files	Lines	Code	Comments	Blanks	Complexity	Bytes
Go	29	2931 (+1)	2149 (+1)	280	502	392	76.5K (+44B)
Go (test)	7	1505 (+6)	1209 (+5)	46	250 (+1)	84	42.8K (+586B)

[github-actions]

Benchmark Result

Benchmark diff with base branch

name     old time/op    new time/op    delta
Queue-2     234µs ±11%     231µs ± 8%   ~     (p=0.818 n=6+6)

name     old alloc/op   new alloc/op   delta
Queue-2    35.6kB ± 8%    35.7kB ±11%   ~     (p=0.818 n=6+6)

name     old allocs/op  new allocs/op  delta
Queue-2      87.5 ± 1%      87.7 ± 1%   ~     (p=1.000 n=6+6)

Benchmark result

name     time/op
Queue-2   231µs ± 8%

name     alloc/op
Queue-2  35.7kB ±11%

name     allocs/op
Queue-2    87.7 ± 1%

[github-actions]

Go API Changes

# summary
Inferred base version: v5.1.0
Suggested version: v5.1.1

[vearutop]

Issue reproduced in test run: https://github.com/adjust/rmq/actions/runs/4126465190/jobs/7128426003.

[github-actions]

Unit Test Coverage

total: (statements) 71.1%
changed lines: (statements) 83.3%, coverage is less than 90.0%, consider testing the changes more thoroughly

Coverage of changed lines

File	Function	Coverage
Total		83.3%
delivery.go		83.3%
delivery.go:39	newDelivery	100.0%
delivery.go:67	String	100.0%
delivery.go:71	Payload	100.0%

Coverage diff with base branch

File	Function	Base Coverage	Current Coverage
Total		70.9%	71.1% (+0.2%)
github.com/adjust/rmq/v5/delivery.go	String	0.0%	100.0% (+100.0%)