Fix onSize callback called whether or not size has changed

[felthy]

I noticed that the hasSizeChanged() check was always returning true when in callback mode, because it was comparing to this.state which never changes from its initial state. Changed to compare to this.strategisedGetState() instead, which also necessitated initialising the callback state which I did inside determineStrategy().

All tests passing.

[codecov]

Codecov Report

Merging #116 into master will increase coverage by 0.3%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #116     +/-   ##
=========================================
+ Coverage   93.67%   93.97%   +0.3%     
=========================================
  Files           3        3             
  Lines          79       83      +4     
  Branches       24       25      +1     
=========================================
+ Hits           74       78      +4     
  Misses          2        2             
  Partials        3        3

Impacted Files	Coverage Δ
src/sizeMe.js	100% <100%> (ø)	⬆️

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[ctrlplusb]

Awesome dude thanks for this!