Add tests that show #409

[jvican]

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[jvican]

Apparently, both the pickler and unpickler that are previously returned from lookup{Un}Pickler are created by a generator stored in genPicklerMap. I'm actually debugging why this is happening in the first place and where we put that generator (which is a generator of List). Also, I guess that these generators are used only because of reflection and we do use them also for generating static picklers, but we shouldn't. And if we do, we should cache them correctly and make them visible so that implicitly[Pickler[List[String]]] doesn't trigger the standard generation. I'm trying to find a slick way to solve this issue.

[jvican]

My diagnosis: when we look a pickler up, it doesn't only look it up in the picklerMap but the genPicklerMap. Before querying the last one, we extract the top level type and use it as the key, e.g. from List[String] we get List. Then, the lookup succeeds and returns the generator for List, which doesn't correspond to Pickler[List[String]]].

To solve this problem it's necessary to register the picklers/unpicklers that are created from the generators. Therefore, next time someone looks them up they will be in picklerMap. Also, I've added two new methods in DefaultPicklerRegistry that only query picklerMap/unpicklerMap, ignoring the generators.

[jvican]

Tests passing thanks to #411. Review by @jsuereth.