fix: strip spaces from the operator when translating nth css to xpath

[fusion2004]

What problem is this PR intended to solve?

Prevent errors when nokogiri is translating nth css to xpath and there are spaces in it. (E.g. 2n + 1 instead of 2n+1)

Background: This came up because prettier started rewriting my email css from li:nth-child(2n+1) to li:nth-child(2n + 1) (which is perfectly valid in browsers) and this caused a expected an+b node to have either + or - as the operator, but is " + " (ArgumentError) error from https://github.com/Mange/roadie using nokogiri as part of inlining the css.

Have you included adequate test coverage?

I believe so!

Does this change affect the behavior of either the C or the Java implementations?

Since I only modified ruby code, I'm assuming not?

[flavorjones]

@fusion2004 Thank you for this! Looks good, merging.