How does row style relates to rich text?

[sineed]

Hi! Thank you for awesome gem.

I have one question about usage of rich text. In my project I want to use superscript Excel text feature. Looking for this task led me to this issue #225. For now my implementation looks something like this:

def build_unit(base, exponent)
  unit = Axlsx::RichText.new
  unit.add_run(base)
  unit.add_run(exponent, vertAlign: :superscript)
  unit
end

Then I use this build method in add_row call:

worksheet.add_row(
  [list_number, label, build_unit("m", "3"), value]
  style: [style[:list_number], style[:label], style[:unit], style[:value]]
)

The problem is that style[:unit] doesn't apply to RichText object.

I doesn't know is this a correct behaviour or not, so I wrote two specs which show my assumptions of how this should work. You can see them in this PR #398.

I assume that I can use axlsx api improperly so maybe there is a better solution of assigning rich text run properties from row style.