Preserve API group name when explicitly set

Author: Chris Martinez

src/Microsoft.AspNetCore.Mvc.Versioning.ApiExplorer/VersionedApiDescriptionProvider.cs

@@ -108,7 +108,11 @@ public virtual void OnProvidersExecuted( ApiDescriptionProviderContext context )
 
                     var groupResult = result.Clone();
 
-                    groupResult.GroupName = groupName;
+                    if ( string.IsNullOrEmpty( groupResult.GroupName ) )
+                    {
+                        groupResult.GroupName = groupName;
+                    }
+
                     groupResult.SetApiVersion( version );
                     PopulateApiVersionParameters( groupResult, version );
                     groupResult.TryUpdateRelativePathAndRemoveApiVersionParameter( Options );