Fix compile errors in netstandard

Author: Matthias Gessinger

src/Client/src/Asp.Versioning.Http.Client/System.Net.Http/HttpResponseMessageExtensions.cs

@@ -90,10 +90,19 @@ public static DeprecationPolicy ReadDeprecationPolicy( this HttpResponseMessage
             foreach ( var value in values )
             {
                 var split = value.Trim( '@' );
-                if ( long.TryParse( split, out var unixTimestamp ) &&
-                     ( date == default || date < DateTimeOffset.FromUnixTimeSeconds( unixTimestamp ) ) )
+                if ( long.TryParse( split, out var unixTimestamp ) )
                 {
-                    date = DateTimeOffset.FromUnixTimeSeconds( unixTimestamp );
+                    DateTimeOffset parsed;
+#if NETSTANDARD
+                    parsed = new DateTime(1970, 1, 1) + TimeSpan.FromSeconds(unixTimestamp);
+#else
+                    parsed = DateTimeOffset.FromUnixTimeSeconds( unixTimestamp );
+#endif
+
+                    if ( date == default || date < parsed )
+                    {
+                        date = parsed;
+                    }
                 }
             }
 

src/Client/test/Asp.Versioning.Http.Client.Tests/System.Net.Http/HttpClientExtensionsTest.cs

@@ -41,6 +41,7 @@ public async Task get_api_information_async_should_return_expected_result()
                 {
                     Type = "text/html",
                 } ),
+                new(),
                 new Dictionary<ApiVersion, Uri>() { [new( 1.0 )] = new( "http://tempuri.org/swagger/v1/swagger.json" ) } ) );
     }
 }