Test case for dechex conversion & fix

After investigating #25 , it turns out that :

* dechex is not used anywhere in the project, thus the change can be considered safe
* GMP and BCMath (despite BCMath not being recommended) did not have consistent behavior for dechex

Hence this P/R, going back on my comment.
Oops, more than empty comment, empty test. That file should not have been in the commit dropping it