Update PangramTest.fs
Update PangramTest.fs

Looking a solutions at (http://exercism.io/tracks/fsharp/exercises/pangram) I found one that would pass all the unit test and yet be invalid since it does not check whether symbols belong to alphabet 'a'..'z'
exercism/xfsharp,ErikSchierboom/fsharp,ErikSchierboom/xfsharp