conditional signature keyword arguments

[t-bltg]

Fix https://github.com/gcalderone/ReusePatterns.jl/issues/5.

(at least remove the remaining error on the issue example).

• [x] add tests

[t-bltg]

@gcalderone, this is a shame the repo doesn't have CI, do you want me to add it ?

EDIT: added, local tests are green.