make Autosizer not pure since it always re-renders anyway because it receives children as function

[vzaidman]

Since the suggested use of the AutoSizer is passing an inline function to it, it doesn't makes sense the AutoSizer component is pure, since it would re-render anyway:

  <AutoSizer>
    {({height, width}) => (
      // this is a new function on each render.
    )}
  </AutoSizer>,

If anybody wants to make it pure, they still can:

const PureAutoSizer = React.memo(AutoSizer);

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  Should i document it?
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[codecov-io]

Codecov Report

Merging #1490 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1490   +/-   ##
=======================================
  Coverage   94.66%   94.66%           
=======================================
  Files          47       47           
  Lines        1801     1801           
=======================================
  Hits         1705     1705           
  Misses         96       96

Impacted Files	Coverage Δ
source/AutoSizer/AutoSizer.js	100% <ø> (ø)	⬆️

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[wuweiweiwu]

although this problem can be alleviated by pulling the function outside it makes passing props to the render prop more complex.

Thanks for the contribution!