check if file responds to filename as well as to_path

[lsylvester]

After upgrading to rails 5 and sprockets 3.5.2, Compass::Core::SassExtensions::Functions::ImageSize::ImageProperties is being initialised with a Sprockets::Asset which no longer responds to to_path.

This adds an additional check to to see the file passed in responds to filename and uses that if it is available to prevent the no implicit conversion of Sprockets::Asset into String errors.

Fixes #2042