Fix a flaky test testUserController

[dserfe]

This PR is to fix a flaky test com.didispace.chapter21.Chapter21ApplicationTests#testUserController in module 2.x/chapter2-1, we found it when using the latest version of SpringBoot-Learning:

1. To reproduce test failures:

• Run the following cmds:

 mvn edu.illinois:nondex-maven-plugin:2.1.1:nondex -pl 2.x/chapter2-1 -Dtest=com.didispace.chapter21.Chapter21ApplicationTests#testUserController -DnondexRuns=10

• Then we'll get failures:

Chapter21ApplicationTests.testUserController:52 Response content
Expected: "[{\"id\":1,\"name\":\"测试大师\",\"age\":20}]" but: was "[{\"age\":20,\"name\":\"测试大师\",\"id\":1}]"

2. Why it fails:
   Line 52 of 2.x/chapter2-1/src/test/java/com/didispace/chapter21/Chapter21ApplicationTests.java converts a JSON object to a string. Note that this conversion does not guarantee the order of the elements.

3. Fix:
   Use json() to compare Json objects instead of converting them to string.

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