refactor: post data with body

convert_test.go

@@ -39,7 +39,7 @@ var demoTestCase = &TestCase{
 			AssertEqual("body.args.foo2", "34.5", "check args foo2"),             // notice: request params value will be converted to string
 		Step("post json data").
 			POST("/post").
-			WithJSON(map[string]interface{}{
+			WithBody(map[string]interface{}{
 				"foo1": "$varFoo1",       // reference former extracted variable
 				"foo2": "${max($a, $b)}", // 12.3; step level variables are independent, variable b is 3.45 here
 			}).