In combine_words, when words vector is length 2 and and is blank, use sep

[cderv]

This fixes #2044

@yihui I don't know if this is the fix you were expected.

The behavior described in #2044 is currently the one documented:

When words is of length 2, the first word and second word are combined using the and string.

knitr::combine_words(c("a", "b"), before = "`", and = "")
#> `a``b`
knitr::combine_words(c("a", "b"), before = "`", and = " or ")
#> `a` or `b`

What I am proposing here is that we support a special case: When length is two AND knitr::is_blank(and), then we use sep

knitr::combine_words(c("a", "b"), before = "`", and = "")
#> `a`, `b`
knitr::combine_words(c("a", "b"), before = "`", and = " or ")
#> `a` or `b`

Is that ok ?

[yihui]

That sounds good to me. Thank you!