[Rd] bugs in head() and tail()

[Abs Spurdle]

(Using R 3.5.3).

I found bugs in head() and tail().

The following works:

> f = function () 1
> head (f)
1 function ()
2 1

However, the following does not:

> class (f) = "f"
> head (f)
Error in x[seq_len(n)] : object of type 'closure' is not subsettable

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