Fix/2 nested serializer calls

In case of serializer recursion, the serializer send an exception: "Can't pop from an empty datastructure", as noted by @damienalexandre in schmittjoh/serializer#319 

This PR allows to stack the navigator association to visitors, thus making it possible to nest serializer calls. It contains two commits:
 * the first one contains the required changes for the bug to be fixed;
 * the second one adds a test which shows up the bug resolution (tested against the fixed testsuite proposed in schmittjoh/serializer#292)