Fix JSON Parsing Issues in CodableTransform

[Woollim]

Why

1. Problem occurred when trying to parse the JSON Array.

let json = ["array_value": [1, 2, 3, 4, 5]]
// not working! 🙀
self.array = try map.value("array_value", using: CodableTransform<[Int]>())

2. Problem occurred when trying to parse single JSON value using Key Path.

let json = ["int_value": 1]
// not working too! 🙀
self.value = try map.value("int_value", using: CodableTransform<Int>())

What

1. Fix transformFromJSON function logic in CodableTransform.swift
2. Add test case for JSON Array, Single Value From JSON Object parsing

[Woollim]

I wrote Single Value parsing code by referring to Moya's code.
Link: Moya/Response.swift/Line-167