unbox value class in Map key when serializing

[k163377]

This PR is a continuation of #468, and a fix for #469.
This change will also cause unboxing when value class is set as key.
The tests now reflect this as well.

Repletion

About the behavior when WrapperClass is set to key

#468 described a pattern that would unbox the value class field when the WrapperClass was set to key.
On the other hand, this appeared to be a behavior that improperly overrides the toString method of the WrapperClass, so it has been removed.

Fixed NullValueSerializer being ignored when unbox result is null

In the previous PR, there was a problem that null was written ignoring the NullValueSerializer when the result of unbox was null.
Test cases have been added as well.

Question

The JSON string output by writerWithDefaultPrettyPrinter respects the newline code of the platform, but the Kotlin raw string always seems to use \n.
This is why the Github464 test is failing on my Windows environment.

How should I fix this problem?
I believe that replacing \r\n with \n for both output results would superficially solve the problem, but I'm wondering if there is a better fix.

[dinomite]

I think the line separator is because the raw strings are so raw as to abide by the encoding of the source file, and therefore use the \n therein, since that test file is in Unix format. I'm not sure of an elegant solution to this, aside from building the strings using .lineSeparator(), but that's ugly. They'll pass in CI, which is the important thing.