Meta title always get the site name appended

Problem/Motivation

In html.html.twig, drupal does the following
<title>{{ head_title|safe_join(' | ') }}</title>

But the thing is that metatag only alters head_title.title and let the core's head_title.name and head_title.slogan values so the site name is always appended to the meta title even when no [site:name] token is used.

Steps to reproduce :

1. Install drupal
2. Enable metatag
3. Add the metatag field on your content type
4. Use [node:title] | [site:name] as default value as adviced by the help text
5. Create a node

Expected result : [node:title] | [site:name]
Current result : [node:title] | [site:name] | [site:name]

Proposed resolution

Clear the head_title array before adding the title.

Remaining tasks

Patch, review, commit.

User interface changes

None.

API changes

None.

Data model changes

None.