Fix for Nullable.

Source/MQTTnet/Extensions/UserPropertyExtension.cs

@@ -1,4 +1,5 @@
 using System;
+using System.ComponentModel;
 using System.Linq;
 
 namespace MQTTnet.Extensions
@@ -17,9 +18,10 @@ namespace MQTTnet.Extensions
         {
             var value = GetUserProperty(message, propertyName, comparisonType);
 
+            var typeDescriptor = TypeDescriptor.GetConverter(typeof(T));
             try
             {
-                return (T) Convert.ChangeType(value, typeof(T));
+                return (T) typeDescriptor.ConvertFromString(value);
             }
             catch (Exception ex)
             {

Source/MQTTnet/MQTTnet.csproj

@@ -42,6 +42,7 @@
   </PropertyGroup>
 
   <ItemGroup Condition="'$(TargetFramework)'=='netstandard1.3'">
+    <PackageReference Include="System.ComponentModel.TypeConverter" Version="4.3.0" />
     <PackageReference Include="System.Net.Security" Version="4.3.2" />
     <PackageReference Include="System.Net.WebSockets" Version="4.3.0" />
     <PackageReference Include="System.Net.WebSockets.Client" Version="4.3.2" />