Harden RPC support classes - #2000

On branch mergify/bp/v5.x/pr-2000
Your branch is ahead of 'origin/v5.x' by 1 commit.
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You are currently cherry-picking commit f85d038d.
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Changes to be committed:
	modified:   src/test/java/com/rabbitmq/client/AbstractJsonRpcTest.java

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   src/main/java/com/rabbitmq/tools/jsonrpc/JsonRpcClient.java