work around backslashes in paths that mono 6 can't handle

Index: src/Content/ContentManager.cs
--- src/Content/ContentManager.cs.orig
+++ src/Content/ContentManager.cs
@@ -219,7 +219,7 @@ namespace Microsoft.Xna.Framework.Content
 				}
 			}
 			// Load the asset.
-			result = ReadAsset<T>(assetName, null);
+			result = ReadAsset<T>(key, null);
 			loadedAssets[key] = result;
 			return result;
 		}
