RETURNS_SELF: Avoids returning mock when mock type is assignable to method return type, but method return type is Object.

[mlichtblau]

Fixes #2686
This PR adds a test which is expected to pass.
Without the change to TriesToReturnSelf it throws a java.lang.ClassCastException.

org.mockitousage.stubbing.StubbingReturnsSelfTest > should_not_fail_when_calling_method_with_generic_return_type FAILED
    java.lang.ClassCastException: class org.mockitousage.stubbing.StubbingReturnsSelfTest$Builder$MockitoMock$idXgt9U6 cannot be cast to class java.lang.String (org.mockitousage.stubbing.StubbingReturnsSelfTest$Builder$MockitoMock$idXgt9U6 is in unnamed module of loader 'app'; java.lang.String is in module java.base of loader 'bootstrap')
        at org.mockitousage.stubbing.StubbingReturnsSelfTest.should_not_fail_when_calling_method_with_generic_return_type(StubbingReturnsSelfTest.java:80)

[codecov-commenter]

Codecov Report

Merging #2687 (8a53c3e) into main (ec27830) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##               main    #2687   +/-   ##
=========================================
  Coverage     86.26%   86.26%           
- Complexity     2776     2780    +4     
=========================================
  Files           315      315           
  Lines          8334     8334           
  Branches       1037     1035    -2     
=========================================
  Hits           7189     7189           
  Misses          874      874           
  Partials        271      271           

Impacted Files	Coverage Δ
...nal/stubbing/defaultanswers/TriesToReturnSelf.java	100.00% <100.00%> (ø)

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[TimvdLippe]

Makes a lot of sense, nice fix!