Improve prettiness of printing HTML attributes by DocPretty

Multiple attributes should be separated by whitespace:

@@ -497,7 +492,7 @@ public class DocPretty implements DocTreeVisitor<Void,Void> {
             List<? extends DocTree> attrs = node.getAttributes();
             if (!attrs.isEmpty()) {
                 print(" ");
- print(attrs);
+ print(attrs, " ");
                 DocTree last = node.getAttributes().get(attrs.size() - 1);
                 if (node.isSelfClosing() && last instanceof AttributeTree
                         && ((AttributeTree) last).getValueKind() == ValueKind.UNQUOTED)