periods between names in @family attribute

[kevinushey]

When a package contains only one unique @family tag, e.g.

@family scope-related functions

roxygen2 mutates the name to

scope.related.functions

This seems to be caused by the definition of invert. A small reproducible example:

data <- structure(list(defer.Rd = "scope-related functions", scope_dir.Rd = "scope-related functions", 
    scope_env_vars.Rd = "scope-related functions", scope_options.Rd = "scope-related functions"), .Names = c("defer.Rd", 
"scope_dir.Rd", "scope_env_vars.Rd", "scope_options.Rd"))
roxygen2:::invert(data)

This could be fixed by avoiding the use of stack(), unstack() in invert: https://github.com/klutometis/roxygen/blob/master/R/family.R#L29-L32