Function name not passed to LoadFrame method when file name is not present in stacktrace

When file names are missing from exception stack trace the above line of code does not pass the detected function name. As a result issue described in bug : https://github.com/aspnet/Diagnostics/issues/49 happens.

This issue happens on mono. With this fix method names show up in the error page middleware output on mono.

src/Microsoft.AspNet.Diagnostics/ErrorPageMiddleware.cs

@@ -142,7 +142,7 @@ namespace Microsoft.AspNet.Diagnostics
             int lineNumber = line.ToInt32();
 
             return string.IsNullOrEmpty(file)
-                ? LoadFrame(line.ToString(), string.Empty, 0, showSource)
+                ? LoadFrame(function, string.Empty, 0, showSource)
                 : LoadFrame(function, file, lineNumber, showSource);
         }