GetETag: don't ignore weakness indicator (#8)

* GetETag: don't use .let too much, add tests

* Added `weak` attribute

Signed-off-by: Arnau Mora <arnyminer.z@gmail.com>

* Added tests for `weak` attribute

Signed-off-by: Arnau Mora <arnyminer.z@gmail.com>

* Added tests for `weak` attribute

Signed-off-by: Arnau Mora <arnyminer.z@gmail.com>

* Documented `eTag` and `weak`

Signed-off-by: Arnau Mora <arnyminer.z@gmail.com>

* Refactor `weak` setting

Signed-off-by: Arnau Mora <arnyminer.z@gmail.com>

* Using JUnit's `assertTrue`

Signed-off-by: Arnau Mora <arnyminer.z@gmail.com>

* Use assertTrue / assertFalse

Signed-off-by: Arnau Mora <arnyminer.z@gmail.com>
Co-authored-by: Ricki Hirner <hirner@bitfire.at>