MemberNameEqualsClassName: fix false negative when function has no explicit return type

[t-kameyama]

Fixes #655

[codecov-io]

Codecov Report

Merging #2502 into master will increase coverage by 0.02%.
The diff coverage is 88.88%.

@@             Coverage Diff              @@
##             master    #2502      +/-   ##
============================================
+ Coverage     80.03%   80.05%   +0.02%     
- Complexity     2255     2257       +2     
============================================
  Files           378      378              
  Lines          6621     6628       +7     
  Branches       1176     1178       +2     
============================================
+ Hits           5299     5306       +7     
  Misses          740      740              
  Partials        582      582

Impacted Files	Coverage Δ	Complexity Δ
...h/detekt/rules/naming/MemberNameEqualsClassName.kt	90.9% <88.88%> (+2.44%)	16 <2> (+2)	⬆️

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[schalkms]

Thanks for contributing another awesome fix!
Great work again! I'm waiting a few hours for a second look over this PR.
Then I'd say LGTM! :-)