Fix LaTeX error caused by \midrule interference

[rich-iannone]

This PR solves an issue where some inputs following a \midrule are mistakenly interpreted as \midrule input. Previous to these changes, simple LaTeX tables would fail on certain characters following a midrule line. This set of examples (when rendered to PDF through any LaTeX engine) shows which cases render successfully and which fail.

# Works
gt::gt(data.frame(A = c("\u00A0(OK)", "sdf")))
gt::gt(data.frame('a' = c('OK', '(OK)'), 'b' = 1:2))
gt::gt(data.frame(B = "OK", A = "(OK)"))

# Fails
gt::gt(data.frame(A = "(BAD)"))
gt::gt(data.frame(A = paste0(knitr::asis_output("\u0028"), "BAD)")))
gt::gt(data.frame(A = "\u0028 BAD)"))
gt::gt(data.frame('a' = c('(BAD)', 'OK'), 'b' = 1:2))
gt::gt(data.frame(B = "(BAD)", A = "OK"))
gt::gt(data.frame(A = c("  (BAD)", "sdf")))

Which the changes made here, the previously failing cases now all work.

Fixes: #145
Fixes: #391

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