non-ASCII characters in directly passed header/footer not working unless LANG is set

[CherryDT]

On Linux, passing for example footerCenter: 'abcäöüdef' won't work and will put abcdef into the footer unless the LANG environment variable is set.

I suggest that this library should spawn wkhtmltopdf with LANG=C.UTF-8 set.

My current workaround is:

wkhtmltopdf(input, {
  ...options,
  spawnOptions: { env: { LANG: 'C.UTF-8' } } // << THIS
})

(I discovered spawnOptions by looking at the source code, it's not documented.)