HTTP: Added variable validation to the response_headers option

[hongzhidao]

This is to improve error messages for response headers configuration. Take the configuration as an example:

  {
      "response_headers": {
          "a": "$b"
      }
  }

Previously, when applying it the user would see this error message:

  failed to apply previous configuration

After this change, the user will see this improved error message:

  the previous configuration is invalid: Unknown variable "b" in the "a" value