Fix nil consumer group

[kasperbe]

Closes #209

[codecov]

Codecov Report

Merging #208 into master will decrease coverage by 0.13%.
The diff coverage is 84.81%.

@@            Coverage Diff             @@
##           master     #208      +/-   ##
==========================================
- Coverage   84.48%   84.35%   -0.14%     
==========================================
  Files          52       52              
  Lines        3074     3093      +19     
==========================================
+ Hits         2597     2609      +12     
- Misses        398      402       +4     
- Partials       79       82       +3     

Impacted Files	Coverage Δ
client/amqp/amqp.go	40.98% <ø> (ø)
client/amqp/option.go	66.66% <ø> (ø)
client/amqp/user.pb.go	26.92% <ø> (ø)
client/http/option.go	100.00% <ø> (ø)
client/redis/redis.go	44.44% <ø> (ø)
client/sns/message.go	97.53% <ø> (ø)
client/sns/publisher.go	93.33% <ø> (ø)
client/sql/sql.go	86.25% <ø> (ø)
client/sqs/message.go	92.50% <ø> (ø)
client/sqs/publisher.go	93.33% <ø> (ø)
... and 62 more

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