Fix duplicated prefixed selectors on reformatted CSS

The `already()` check compared `before.selector` to the generated
prefixed selector with strict equality. Generated prefixed selectors
always join the comma list with `', '`, so once an existing prefixed
rule had its selector list reformatted (e.g. one selector per line by a
formatter), the comparison failed and Autoprefixer inserted a duplicate
prefixed rule on every run.

Normalize the compared selector with `list.comma(...).join(', ')` so
whitespace differences in the selector list no longer defeat the
duplication check.

Closes #1497

Author: xianjianlf2

lib/selector.js

@@ -38,10 +38,12 @@ class Selector extends Prefixer {
         return false
       }
 
+      let beforeSelector = list.comma(before.selector).join(', ')
+
       let some = false
       for (let key in prefixeds[this.name]) {
         let prefixed = prefixeds[this.name][key]
-        if (before.selector === prefixed) {
+        if (beforeSelector === prefixed) {
           if (prefix === key) {
             return true
           } else {

test/selector.test.js

@@ -86,6 +86,15 @@ test('finds prefixed even if unknown prefix is between', () => {
   is(selector.already(css.nodes[2], prefixeds, '-moz-'), true)
 })
 
+test('finds prefixed even if selector list was reformatted', () => {
+  let css = parse(
+    '.a::-moz-selection,\n.b::-moz-selection {}\n' +
+      '.a::selection,\n.b::selection {}'
+  )
+  let prefixeds2 = selector.prefixeds(css.nodes[1])
+  is(selector.already(css.nodes[1], prefixeds2, '-moz-'), true)
+})
+
 test('adds prefix to selectors', () => {
   equal(
     selector.replace('body ::selection, input::selection, a', '-ms-'),