Themes: margin spacing does not adjust correctly for expressions

[grantmcdermott]

Originally reported by @zeileis in #549 (comment)

library(tinyplot)
tinyplot(
  1:10,
  xlab = expression(frac(Delta*numerator, Delta*denominator)),
  ylab = expression(Precipitation~"["~mm^{1/2}~"]"),
  theme = "clean2"
)