Enable tests for .net core 3.1 and 2.2

Author: JohnnyCrazy

SpotifyAPI.Web.Tests/Http/APIConnectorTest.cs

@@ -76,7 +76,7 @@ public async Task RetryHandler_CanRetry()
           It.IsAny<IResponse>(),
           It.IsAny<IRetryHandler.RetryFunc>()
         )
-      ).Returns((IRequest request, IResponse response, IRetryHandler.RetryFunc retry) => retry(request));
+      ).Returns((IRequest request, IResponse _, IRetryHandler.RetryFunc retry) => retry(request));
 
       var apiConnector = new APIConnector(
         new Uri("https://spotify.com"),

SpotifyAPI.Web.Tests/SpotifyAPI.Web.Tests.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>netcoreapp3.1</TargetFramework>
+    <TargetFrameworks>netcoreapp3.1;netcoreapp2.2</TargetFrameworks>
     <IsPackable>false</IsPackable>
   </PropertyGroup>