Add canonical link to PREFIX_AND_DEFAULT duplicated pages

[Nabellaleen]

When using prefix_and_default strategy, the default language pages have "duplicated routes". For example, if I have a route A/B with en as default language, I will get /en/A/B and /A/B routes.

For SEO, we needs, then, a rel="canonical" link.
This MR add on the /en/A/B route a canonical link to the /A/B route

More info: https://support.google.com/webmasters/answer/182192#dup-content

PS: I tried to add tests, but I've failed to setup tests on multiple nuxt.config.js files :/

[paulgv]

Thanks you @Nabellaleen