Use the runtime type for a wildcard element type

[HuzaifaChaudary]

Fixes #1870

getRuntimeTypeIfMoreSpecific only looks at the runtime type for a Class or a
TypeVariable . ? extends Base is a WildcardType , so it is neither , and the adapter for
the declared bound is used instead :

List<? extends Base>   ->  {"list":[{"a":"1"}]}            b is missing
List<Base>             ->  {"list":[{"b":"2","a":"1"}]}

same value , same field , different json .

a wildcard says nothing more than its upper bound , so this reads the bound and then decides
the same way it already does for a plain type . the bound is only used to make the decision ,
the declared type is still what gets kept when it is parameterized , so ? extends List<String>
behaves like List<String> and does not lose its type arguments . that is why this reads the
bound rather than just adding || type instanceof WildcardType , which would swap in the raw
runtime class and drop them .

while testing i found a worse case of the same cause that is not in the report .
List<? extends CharSequence> holding a String and a StringBuilder serialized as
[{},{}] , because gson fell back to a reflective adapter for the interface . the strings were
simply gone . List<CharSequence> gives ["s","t"] . that is fixed by the same change .

four tests added , covering a wildcard list , a wildcard map value , and a wildcard reached
through a type variable . the fourth goes the other way and pins that
Collection<? extends ParameterizedBase<String>> still keeps the declared type , so the change
is not too loose . it passes with and without .

4637 tests pass in the gson module , 0 failures , and the full reactor is 4856 across all eight
modules . spotless:check is clean .

one related thing i deliberately left out . registerTypeAdapter(Base.class, adapter) is still
ignored when the element type is ? extends Base , because the lookup token never matches one
registered for Base.class . that needs the wildcard stripped at adapter lookup time , where
there is already a // TODO: strip wildcards? in GsonTypes , and it is a wider change to
your internals , so it seemed better to leave that call to you . this change still improves that
case , the subclass fields stop disappearing .

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[HuzaifaChaudary]

@googlebot I signed it!