feat: `PostResponse` supports charset other than `UTF-8`

[joyyir]

Please answer these questions before submitting a pull request

• Why submit this pull request?

• [ ] Bugfix

• [x] New feature provided

• [ ] Improve performance

• Related issues

  238

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  Bugfix

• Description

• How to fix?

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New feature or improvement

• Describe the details and related test reports. PostResponse by default converts the body to bytes in UTF-8. It would be nice to support other charsets.

• Source branch main

• Related commits and pull requests

• Target branch main

[joyyir]

Added test cases. Thank you.

[nic-chen]

@joyyir thanks!