Fix extra css heading so only added once

R/style_extra_css.R

   has_heading <- !is.null(heading)
   x <- paste0(
     if (has_heading) paste0("/* ", heading, " */\n"),
-    list2css(css)
+    paste(list2css(css), collapse = "\n")
   )
   if (append) x <- paste0(if (has_heading) "\n\n" else "\n", x)
   if (is.null(outfile)) return(x)

tests/testthat/test-style_extra_css.R

       strsplit(third_exp, "\n")[[1]]
     )
   })
+
+  it("only adds prepends `heading` once", {
+    css <- list(b = list(color = "red"), i = list(color = "blue"))
+    out <- style_extra_css(css, outfile = NULL, heading = "Extra CSS")
+    expect_equal(length(out), 1L)
+    expect_equal(sum(grepl("Extra CSS", out)), 1L)
+  })
 })