Optimize functions decorator

[tonybaloney]

Creates an additional stack call for an instance property.

[codecov]

Codecov Report

Merging #129 (66da2e5) into dev (45e16bc) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##              dev     #129   +/-   ##
=======================================
  Coverage   90.09%   90.09%           
=======================================
  Files          51       51           
  Lines        2959     2959           
  Branches      396      396           
=======================================
  Hits         2666     2666           
  Misses        218      218           
  Partials       75       75           

Flag	Coverage Δ
unittests	90.06% <100.00%> (ø)

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Impacted Files	Coverage Δ
azure/functions/decorators/function_app.py	99.40% <100.00%> (ø)

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