[28177] Fix   breaking markdown formatting in turndown

Turndown doesn't replace   before syntax identifiers such as
`<strong>foobar&nbsp;</strong>` will result in `**foobar **` (with the
space being `\u00A0`.

We can fix this by replacing all &nbsp; by CKEeditor beforehand,
knowingly modifying the behavior of the spaces.

Bug report at turndown: mixmark-io/turndown#250

spec/features/wysiwyg/non_breaking_spaces_spec.rb

@@ -0,0 +1,68 @@
+#-- copyright
+# OpenProject is a project management system.
+# Copyright (C) 2012-2018 the OpenProject Foundation (OPF)
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License version 3.
+#
+# OpenProject is a fork of ChiliProject, which is a fork of Redmine. The copyright follows:
+# Copyright (C) 2006-2017 Jean-Philippe Lang
+# Copyright (C) 2010-2013 the ChiliProject Team
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+#
+# See docs/COPYRIGHT.rdoc for more details.
+#++
+
+require 'spec_helper'
+
+describe 'Wysiwyg   behavior',
+         type: :feature, js: true do
+  let(:user) { FactoryBot.create :admin }
+  let(:project) { FactoryBot.create(:project, enabled_module_names: %w[wiki]) }
+  let(:editor) { ::Components::WysiwygEditor.new }
+
+  before do
+    login_as(user)
+  end
+
+  describe 'in wikis' do
+    describe 'creating a wiki page' do
+      before do
+        visit project_wiki_path(project, :wiki)
+      end
+
+      it 'can insert strong formatting with nbsp' do
+        editor.in_editor do |container, editable|
+
+          editor.click_and_type_slowly 'some text '
+          container.find('.ck-button', visible: :all, text: 'Bold').click
+
+          editor.click_and_type_slowly 'with bold '
+        end
+
+        # Save wiki page
+        click_on 'Save'
+
+        expect(page).to have_selector('.flash.notice')
+
+        within('#content') do
+          expect(page).to have_selector('p', text: 'some text with bold')
+          expect(page).to have_selector('strong', text: 'with bold')
+        end
+      end
+    end
+  end
+end