leading space in clean_names becomes an x

[sfirke]

> dat <- data.frame(1)
> names(dat)[1] <- " my interview"
> clean_names(dat)
  x_my_interview
1              1

Preferably would just be stripped off.

[sfirke]

my fix was simple, it handles spaces only - other leading illegal name characters will still cause make.names() to turn them into x. Say, the above example but with a leading !. But, (a) those are rarer cases and (b) there is arguably meaning encoded in non-space leading characters like an asterisk or exclamation point that is retained by the x. So this is good enough for now.