Throw an error if c(font_string, google_font())

Closes #19

R/style_extra_css.R

 }
 
 list2fonts <- function(fonts) {
+  if (
+    length(setdiff(names(google_font('fam')), names(fonts))) == 0 &&
+    !inherits(fonts, "google_font")
+  ) {
+    # concatenating a string and a google_font() provides a wacky list
+    stop(
+      "Multiple fonts in `extra_fonts` must be specified inside a `list()`.",
+      call. = FALSE
+    )
+  }
   if (inherits(fonts, "google_font")) {
     fonts <- list(fonts)
   }

tests/testthat/test-write_extra_css.R

   lato_url <- "https://fonts.googleapis.com/css?family=Lato"
   worksans_url <- "https://fonts.googleapis.com/css?family=Work+Sans"
 
-  it("handles a list of font names", {
+  it("handles a list or c() of font urls", {
     expect_equal(
       list2fonts(list(lato_url, worksans_url)),
       import(c(lato_url, worksans_url))
     )
+    expect_equal(list2fonts(c(lato_url, lato_url)), rep(import(lato_url), 2))
   })
 
   it("handles single character font name", {
       import(lato_url)
     )
   })
+
+  it("throws an error when c() used to combine string and google_font()", {
+    expect_error(
+      list2fonts(c(lato_url, google_font("Lato"), google_font("Work Sans"))),
+      "Multiple fonts"
+    )
+    expect_error(
+      list2fonts(c(google_font("Lato"), google_font("Work Sans"))),
+      "Multiple fonts"
+    )
+  })
 })