Fix coding style

Author: ishkawa

APIKit/URLEncodedSerialization.swift

@@ -1,12 +1,16 @@
 import Foundation
 
 private func escape(string: String) -> String {
-    //Reserved characters defined by RFC 3986
-    let genDelims = ":/?#[]@"
-    let subDelims = "!$&'()*+,;="
-    let reservedCharacters = genDelims + subDelims
-    let allowedCharacterSet = NSCharacterSet.URLQueryAllowedCharacterSet().mutableCopy() as! NSMutableCharacterSet
+    // Reserved characters defined by RFC 3986
+    // Reference: https://www.ietf.org/rfc/rfc3986.txt
+    let generalDelimiters = ":/?#[]@"
+    let subDelimiters = "!$&'()*+,;="
+    let reservedCharacters = generalDelimiters + subDelimiters
+    
+    let allowedCharacterSet = NSMutableCharacterSet()
+    allowedCharacterSet.formUnionWithCharacterSet(NSCharacterSet.URLQueryAllowedCharacterSet())
     allowedCharacterSet.removeCharactersInString(reservedCharacters)
+    
     return string.stringByAddingPercentEncodingWithAllowedCharacters(allowedCharacterSet) ?? string
 }