Normalize attribute names on creation

Allows preserved case attributes in input to make it into the output, even if the safelist attribute name was set as lowercase).

Fixes attribute case in #2049

src/main/java/org/jsoup/safety/Safelist.java

@@ -632,7 +632,7 @@ static class AttributeKey extends TypedValue {
         }
 
         static AttributeKey valueOf(String value) {
-            return new AttributeKey(value);
+            return new AttributeKey(Normalizer.lowerCase(value));
         }
     }
 

src/test/java/org/jsoup/safety/CleanerTest.java

@@ -407,18 +407,19 @@ public void bailsIfRemovingProtocolThatsNotSet() {
     @ParameterizedTest @ValueSource(booleans = {true, false})
     void cleansCaseSensitiveElements(boolean preserveCase) {
         // https://github.com/jhy/jsoup/issues/2049
-        String html = "<svg><feMerge><feMergeNode kernelMatrix=1 /><feMergeNode><clipPath /></feMergeNode><feMergeNode />";
+        String html = "<svg><feMerge baseFrequency=2><feMergeNode kernelMatrix=1 /><feMergeNode><clipPath /></feMergeNode><feMergeNode />";
         String[] tags = {"svg", "feMerge", "feMergeNode", "clipPath"};
-        String[] attrs = {"kernelMatrix"};
+        String[] attrs = {"kernelMatrix", "baseFrequency"};
 
         if (!preserveCase) {
             tags = Arrays.stream(tags).map(String::toLowerCase).toArray(String[]::new);
+            attrs = Arrays.stream(attrs).map(String::toLowerCase).toArray(String[]::new);
         }
 
         Safelist safelist = Safelist.none().addTags(tags).addAttributes(":all", attrs);
         String clean = Jsoup.clean(html, safelist);
         String expected = "<svg>\n" +
-            " <feMerge>\n" +
+            " <feMerge baseFrequency=\"2\">\n" +
             "  <feMergeNode kernelMatrix=\"1\" />\n" +
             "  <feMergeNode>\n" +
             "   <clipPath />\n" +