enable different response format #123

[FrenchCommando]

Summary

• preview_equity_order, change_preview_equity_order, place_equity_order, and place_changed_equity_order hardcoded "xml" in their perform_request calls, ignoring the resp_format kwarg passed by
  the caller
• Replaced the hardcoded "xml" with kwargs.get("resp_format", "xml") so the parameter is properly forwarded
• place_option_order and place_changed_option_order also inherit the fix since they delegate via **kwargs

Test plan

• Added 4 new tests that call each fixed method with resp_format="json", mock the session to return JSON, and assert the response is parsed as JSON (not piped through xmltodict)
• All 11 tests pass