removed hash equality from yy_modelIsEqual:

[TarekkMA]

This is a fix for #278 . hash equality won't work if the object doesn't ovrride the hash method.

[codecov-io]

Codecov Report

Merging #279 into master will decrease coverage by 5.32%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #279      +/-   ##
==========================================
- Coverage   99.28%   93.95%   -5.33%     
==========================================
  Files           2        2              
  Lines        1540     1539       -1     
  Branches        0      421     +421     
==========================================
- Hits         1529     1446      -83     
+ Misses         11        7       -4     
- Partials        0       86      +86

Impacted Files	Coverage Δ
YYModel/NSObject+YYModel.m	93.35% <ø> (-5.87%)	⬇️
YYModel/YYClassInfo.m	96.92% <0%> (-2.7%)	⬇️

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