Test accepting thunks passed to pullbacks

[mzgubic]

Partner to JuliaDiff/ChainRules.jl#449

[codecov]

Codecov Report

Merging #73 (f76e190) into master (4cf486b) will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #73      +/-   ##
==========================================
+ Coverage   94.83%   94.90%   +0.07%     
==========================================
  Files           5        5              
  Lines         271      275       +4     
==========================================
+ Hits          257      261       +4     
  Misses         14       14              

Impacted Files	Coverage Δ
src/chainrules.jl	100.00% <100.00%> (ø)

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