Fix CRAN Warning when non-breaking space in default function argument value

[zeehio]

• Closes CRAN Warning when Non-breaking space in default function argument value #1342

I don't have all the context of why there are non-breaking spaces introduced and afterwards replaced. Probably it was a Windows encoding issue already addressed on R-4.2, with its UTF-8 improvements.

The change in this pull request is as follows:

Before:

• The usage Rd section uses a structure like: "<argument_name><space>=<space><default_value>" for each argument
• When default_value contains "\u{A0}" (a non-breaking space character), the generated usage replaces that character with a regular space, leading to a CRAN warning due to the mismatch. (This is a bug, in my opinon)

After:

• The usage Rd section uses a structure like: "<argument_name><space>=<space><default_value>", as before.
• When default_value contains "\u{A0}" (a non-breaking space character), the generated usage preserves that character and does not replace it anymore. May this cause rendering issues on older Windows using locales incompatible with nbsp? Maybe, but I would argue this hasn't happened until now (because if it had happened there would have been a CRAN Warning due to the bug mentioned above) and it probably won't happen after R4.2, since windows builds use UTF-8 there, as far as I know. So I would expect low chances of regressions.

In the future, when R4.2 is required and assuming non-breaking spaces are well supported in R-4.2 Rd files, roxygen2 could use non-breaking spaces to wrap the = sign that separates the argument name from its default value. This is out of the scope of this pull request.