add test for [json:fieldname]

Added test also tests the output from json.encode to make sure the json:fieldname attribute is used.

vlib/json/json_test.v

@@ -1,24 +1,33 @@
 import json
 
 struct User {
-	age       int
-	nums      []int
-	last_name string [json:lastName]
+	age       	int
+	nums      	[]int
+	last_name 	string 	[json:lastName]
+	is_registered	bool 	[json:IsRegistered]
 }
 
 fn test_parse_user() {
-	s := '{"age": 10, "nums": [1,2,3], "lastName": "Johnson"}'
+	s := '{"age": 10, "nums": [1,2,3], "lastName": "Johnson", "IsRegistered": true}'
 	u := json.decode(User, s) or {
 		exit(1)
 	}
 	assert u.age == 10
 	assert u.last_name == 'Johnson'
+	assert u.is_registered == true
 	assert u.nums.len == 3
 	assert u.nums[0] == 1
 	assert u.nums[1] == 2
 	assert u.nums[2] == 3
 }
 
+fn test_encode_user(){
+	usr := User{ age: 10, nums: [1,2,3], last_name: 'Johnson', is_registered: true}
+	expected := '{"age":10,"nums":[1,2,3],"lastName":"Johnson","IsRegistered":true}'
+	out := json.encode(usr)
+	assert out == expected
+}
+
 struct Color {
     space string
     point string [raw]
@@ -32,4 +41,3 @@ fn test_raw_json_field() {
     assert color.point == '{"Y":123}'
     assert color.space == 'YCbCr'
 }
-