Refactactor for compatibilty with earlier .NET versions

src/ILovePDF/Core/RequestHelper.cs

@@ -29,6 +29,7 @@
 using System.Security.Authentication;
 using System.Text;
 using System.Threading.Tasks;
+using iLovePdf.Core;
 
 namespace iLovePdf.Core
 { 

src/ILovePDF/Core/iLovePdfApi.cs

@@ -1,5 +1,6 @@
 using System;
 using iLovePdf.Model.Task;
+using iLovePdf.Core;
 
 namespace iLovePdf.Core
 {

src/ILovePDF/Model/TaskParams/Sign/Elements/BaseSignElement.cs

@@ -10,6 +10,7 @@
 using iLovePdf.Model.Enums;
 using System.Xml.Linq;
 using System.Text.RegularExpressions;
+using System.Globalization;
 
 namespace iLovePdf.Model.TaskParams.Sign.Elements
 {

src/ILovePDF/Model/TaskParams/Sign/Elements/InputElement.cs

@@ -1,4 +1,5 @@
 using iLovePdf.Model.Enums;
+using iLovePdf.Model.TaskParams.Sign.Elements;
 using Newtonsoft.Json;
 using System;
 using System.Collections.Generic;

src/ILovePDF/Model/TaskParams/SignParams.cs

@@ -1,7 +1,8 @@
-using iLovePdf.Attributes;
-using iLovePdf.Model.Enums;
+using iLovePdf.Model.Enums;
 using iLovePdf.Model.Task;
 using iLovePdf.Model.TaskParams.Sign.Elements;
+using iLovePdf.Attributes;
+using iLovePdf.Model.TaskParams;
 using Newtonsoft.Json;
 using Newtonsoft.Json.Converters;
 using System;