Fix the thumbnail() method return type

[titouanmathis]

Issue

The returned value of the Post::thumbnail() method is considered to be \Timber\Timber\Image as we already are in the Timber namespace and Timber\Image is missing a leading \. This is making tools such as PHPStan to fail.

Solution

The solution is to use the same type as the Post::get_thumbnail() method, which is directly using the Image class from the current namespace.

Impact

It will improve compatibility with static analysis tools such as PHPStan.

Testing

Nothing to update (I think).

[coveralls]

Coverage increased (+0.0%) to 92.868% when pulling be2c78b on titouanmathis:master into 384f130 on timber:master.

[coveralls]

Coverage increased (+0.0%) to 92.868% when pulling be2c78b on titouanmathis:master into 384f130 on timber:master.

[coveralls]

Coverage remained the same at 92.868% when pulling 8ff4a54 on titouanmathis:master into 8598ce5 on timber:master.

[jarednova]

Thanks @titouanmathis; sorry this has taken me a while — but this looks good and will merge in!