autocomplete: fix case sensitivity (#3854).

ref: https://github.com/r888888888/danbooru/issues/3854#issuecomment-417366253:

app/logical/tag_autocomplete.rb

@@ -10,6 +10,8 @@ module TagAutocomplete
   end
 
   def search(query)
+    query = Tag.normalize_name(query)
+
     candidates = count_sort(
       query,
       search_prefix(query, 3) + 

test/models/tag_autocomplete_test.rb

@@ -3,6 +3,13 @@ require 'test_helper'
 class TagAutocompleteTest < ActiveSupport::TestCase
   subject { TagAutocomplete }
 
+  context "#search" do
+    should "be case insensitive" do
+      create(:tag, name: "abcdef", post_count: 1)
+      assert_equal(["abcdef"], subject.search("A").map(&:name))
+    end
+  end
+
   context "#search_exact" do
     setup do
       @tags = [