fix: treat !important as case-insensitive (#152)

Author: Pixel998

fixtures/ast/declaration/Important.json

@@ -14,9 +14,10 @@
         },
         {
             "source": "property:!ImpoRtant",
+            "generate": "property:!important",
             "ast": {
                 "type": "Declaration",
-                "important": "ImpoRtant",
+                "important": true,
                 "property": "property",
                 "value": {
                     "type": "Value",
@@ -26,9 +27,10 @@
         },
         {
             "source": "property:!IMPORTANT",
+            "generate": "property:!important",
             "ast": {
                 "type": "Declaration",
-                "important": "IMPORTANT",
+                "important": true,
                 "property": "property",
                 "value": {
                     "type": "Value",

lib/syntax/node/Declaration.js

@@ -157,9 +157,10 @@ function getImportant() {
     this.eat(Delim);
     this.skipSC();
 
+    const isImportant = this.cmpStr(this.tokenStart, this.tokenEnd, 'important');
     const important = this.consume(Ident);
 
-    // store original value in case it differ from `important`
-    // for better original source restoring and hacks like `!ie` support
-    return important === 'important' ? true : important;
+    // Store non-standard values for better original source restoring and hacks
+    // like `!ie` support.
+    return isImportant ? true : important;
 }