chopExtension validates path chars twice.

[KevinRansom]

chopextension validates path chars at start and then in has extension. This is probably a waste of cycles.

let hasExtension (s:string) = 
    checkPathForIllegalChars s
    let sLen = s.Length
    (sLen >= 1 && s.[sLen - 1] = '.' && s <> ".." && s <> ".") 
    || Path.HasExtension(s)

let chopExtension (s:string) =
    checkPathForIllegalChars s
    if s = "." then "" else // for OCaml compatibility
    if not (hasExtension s) then 
        raise (System.ArgumentException("chopExtension")) // message has to be precisely this, for OCaml compatibility, and no argument name can be set
    Path.Combine (Path.GetDirectoryName s,Path.GetFileNameWithoutExtension(s))

[KevinRansom]

fixed: #1666

see also: #1677