throw for KMutableEntry.setValue

[passsy]

throw until #55 has been fixed. similar to #6

[codecov]

Codecov Report

Merging #56 into master will increase coverage by 0.93%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master      #56      +/-   ##
==========================================
+ Coverage   96.51%   97.45%   +0.93%     
==========================================
  Files          25       25              
  Lines        1608     1612       +4     
==========================================
+ Hits         1552     1571      +19     
+ Misses         56       41      -15

Impacted Files	Coverage Δ
lib/src/collection/map_mutable.dart	92.85% <100%> (+6.94%)	⬆️
lib/src/collection/iterable.dart	85.71% <60%> (-14.29%)	⬇️
...rc/extension/iterable_mutable_extension_mixin.dart	100% <0%> (+100%)	⬆️

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