Fix #5136: Regular tags are now case-sensitive.

* Fix `AST.tag` to downcase the tag name.
* Change PostQuery::Parser to use build nodes using `AST.tag`,
  `AST.metatag`, `AST.wildcard`, etc methods instead of building nodes
  directly. This way all the normalization happens in the node
  constructor methods instead of in the parser.

test/unit/post_query_builder_test.rb

@@ -85,6 +85,7 @@ class PostQueryBuilderTest < ActiveSupport::TestCase
       post3 = create(:post, tag_string: "bbb ccc")
 
       assert_tag_match([post2, post1], "aaa")
+      assert_tag_match([post2, post1], "AAA")
       assert_tag_match([post2, post1], " aaa ")
     end