Update APIHelper.mustache

This is a fix attempt for : [Swift] Bool value conversion swagger-api#2446 
It is copied from http://stackoverflow.com/questions/36185530/how-to-check-dictionary-value-to-be-exactly-a-bool/36186159#36186159

modules/swagger-codegen/src/main/resources/swift/APIHelper.mustache

@@ -13,6 +13,24 @@ class APIHelper {
             }
         }
 
+        if destination.isEmpty {
+            return nil
+        }
+        return destination
+    }
+    static func convertBoolToString(source: [String: AnyObject]) -> [String:AnyObject]? {
+        var destination = [String:AnyObject]()
+        let theTrue = NSNumber(bool: true)
+        let theFalse = NSNumber(bool: false)
+        for (key, value) in source {
+            switch value {
+                case let x where x === theTrue || x === theFalse:
+                    destination[key] = "\(value as! Bool)"
+                default:
+                    destination[key] = "not a bool"
+            }
+        }
+
         if destination.isEmpty {
             return nil
         }