convert single quotes to double quotes in calc

src/parse/value/css_function.rs

@@ -48,12 +48,12 @@ impl<'a> Parser<'a> {
                     }
                 }
                 q @ '\'' | q @ '"' => {
-                    buf.push(q);
+                    buf.push('"');
                     match self.parse_quoted_string(q)?.node {
                         Value::String(ref s, ..) => buf.push_str(s),
                         _ => unreachable!(),
                     }
-                    buf.push(q);
+                    buf.push('"');
                 }
                 c => buf.push(c),
             }

tests/special-functions.rs

@@ -59,6 +59,11 @@ test!(
     "a {\n  color: calc(\")\");\n}\n",
     "a {\n  color: calc(\")\");\n}\n"
 );
+test!(
+    calc_quoted_string_single_quotes,
+    "a {\n  color: calc('a');\n}\n",
+    "a {\n  color: calc(\"a\");\n}\n"
+);
 test!(
     element_whitespace,
     "a {\n  color: element(       1      );\n}\n",