posts: fix exception when tagging post with char:copy:foo.

Fixup for 015c6dc7d. Show a warning about failure to add a tag instead
of raising an exception when trying to tag a post with `char:copy:foo`.
This tries to create a tag named `copy:foo` then set the category to
character, which doesn't work because `copy:foo` isn't a valid tag name.

app/models/tag.rb

@@ -197,7 +197,7 @@ class Tag < ApplicationRecord
         tag = find_or_create_by(name: normalize_name(name))
 
         if category.present? && current_user.present? && Pundit.policy!(current_user, tag).can_change_category?
-          tag.update!(category: categories.value_for(category))
+          tag.update(category: categories.value_for(category))
         end
 
         tag

test/unit/post_test.rb

@@ -523,28 +523,28 @@ class PostTest < ActiveSupport::TestCase
       end
 
       context "tagged with a metatag" do
-        context "for typing a tag" do
+        context "for a tag category prefix" do
-          setup do
+          should "set the category of a new tag" do
-            @post = FactoryBot.create(:post, tag_string: "char:hoge")
+            create(:post, tag_string: "char:hoge")
-            @tags = @post.tag_array
+
+            assert_equal(Tag.categories.character, Tag.find_by_name("hoge").category)
           end
 
-          should "change the type" do
+          should "change the category for an aliased tag" do
-            assert(Tag.where(name: "hoge", category: 4).exists?, "expected 'moge' tag to be created as a character")
+            create(:tag_alias, antecedent_name: "hoge", consequent_name: "moge")
-          end
+            post = create(:post, tag_string: "char:hoge")
-        end
 
-        context "for typing an aliased tag" do
+            assert_equal(["moge"], post.tag_array)
-          setup do
+            assert_equal(Tag.categories.general, Tag.find_by_name("moge").category)
-            @alias = FactoryBot.create(:tag_alias, antecedent_name: "hoge", consequent_name: "moge")
+            assert_equal(Tag.categories.character, Tag.find_by_name("hoge").category)
-            @post = FactoryBot.create(:post, tag_string: "char:hoge")
-            @tags = @post.tag_array
           end
 
-          should "change the type" do
+          should "not raise an exception for an invalid tag name" do
-            assert_equal(["moge"], @tags)
+            post = create(:post, tag_string: "tagme char:copy:blah")
-            assert(Tag.where(name: "moge", category: 0).exists?, "expected 'moge' tag to be created as a character")
+
-            assert(Tag.where(name: "hoge", category: 4).exists?, "expected 'moge' tag to be created as a character")
+            assert_match(/Couldn't add tag: 'copy:blah' cannot begin with 'copy:'/, post.warnings[:base].join("\n"))
+            assert_equal(["tagme"], post.tag_array)
+            assert_equal(false, Tag.exists?(name: "copy:blah"))
           end
         end