first-last head-tail different in R 4.0

[jangorecki]

@jangorecki seems to be just failing one GLCI job (dev-cran-lin) with:
2 errors out of 9684. Search tests/tests.Rraw for test numbers: 2108.82, 2108.92
Will leave to you for follow-up PR.

Originally posted by @mattdowle in #4065 (comment)

[jangorecki]

R 4.0 introduces a breaking change for head-tail of array.

[jangorecki]

Once I will get confirmation in https://stat.ethz.ch/pipermail/r-devel/2019-December/078827.html I will proceed with a proper fix.