json: fix f32 is not struct

vlib/json/json_test.v

@@ -3,18 +3,22 @@ import json
 struct Employee {
 	name string
 	age  int
+	salary f32
 }
 
 fn test_simple() {
-	x := Employee{'Peter', 28}
+	x := Employee{'Peter', 28, 95000.5}
 	s := json.encode(x)
-	assert s == '{"name":"Peter","age":28}'
+	eprintln('Employee x: $s')
+	assert s == '{"name":"Peter","age":28,"salary":95000.5}'
 	y := json.decode(Employee, s) or {
 		assert false
 		Employee{}
 	}
+	eprintln('Employee y: $y')
 	assert y.name == 'Peter'
 	assert y.age == 28
+	assert y.salary == 95000.5
 }
 
 struct User2 {