TestRestClientInvocation的单元测试无法有效验证功能

问题出在 `TestRestClientInvocation` 的 `testSetCseContext_enable_unicode` 方法和 `testSetCseContext_disable_unicode` 方法中, 两个方法都是开头对`invocation.getContext()`进行mock, 结尾又调用`invocation.getContext()`获取结果进行校验, 获取的结果就是mock返回, 所以达不到测试的预期效果. 1.3.x 和 2.8.x 分支均有此问题, 建议优化一下UT代码.

https://github.com/apache/servicecomb-java-chassis/blob/21d4b4863b71332f151eb63020f4e9ac51583002/transports/transport-rest/transport-rest-client/src/test/java/org/apache/servicecomb/transport/rest/client/http/TestRestClientInvocation.java#L282-L346

Open transport-rest/transport-rest-client/src/test/java/org/apache/servicecomb/transport/rest/client/http/TestRestClientInvocation.java and inspect testSetCseContext_enable_unicode and testSetCseContext_disable_unicode. Start by tracing how invocation.getContext() is mocked, then run these tests and confirm their assertions inspect state produced by the method rather than the mocked return value. Done means both tests meaningfully verify Unicode context behavior on the 1.3.x and 2.8.x branches.

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