Bugfix/issue 552

[francisoliverlee]

redundant consumer group check

What is the purpose of the change

issue fix
#552

Brief changelog

client/src/main/java/org/apache/rocketmq/client/Validators.java checkGroup()
client/src/main/java/org/apache/rocketmq/client/impl/consumer/DefaultMQPullConsumerImpl.java checkConfig()
client/src/main/java/org/apache/rocketmq/client/impl/consumer/DefaultMQPushConsumerImpl.java checkConfig()
client/src/main/java/org/apache/rocketmq/client/impl/producer/DefaultMQProducerImpl.java checkConfig()

Verifying this change

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[coveralls]

Coverage increased (+0.1%) to 43.209% when pulling 40b932f on francisoliverlee:bugfix/issue-552-2018-12-21 into 1bedba8 on apache:master.

[liuruiyiyang]

Maybe we should keep isBlank() check in checkGroup() and add FAQUrl.suggestTodo, and remove the null check after checkGroup(), in order to reduce duplicated code.