Don't add 'shorthand' property to ObjectMethod

[ghost]

Q	A
Bug fix?	yes
Breaking change?	yes (minor)
New feature?	no
Deprecations?	no
Spec compliancy?	yes
Tests added/pass?	Tests modified to pass
Fixed tickets
License	MIT

Assigns shorthand only to ObjectProperty as described in the spec.
Similar to #504.

[codecov]

Codecov Report

Merging #506 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #506   +/-   ##
=======================================
  Coverage   98.26%   98.26%           
=======================================
  Files          22       22           
  Lines        3517     3517           
  Branches      973      973           
=======================================
  Hits         3456     3456           
  Misses         22       22           
  Partials       39       39

Flag	Coverage Δ
#babel	80.94% <100%> (ø)	⬆️
#babylon	97.09% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
src/parser/expression.js	97.38% <100%> (ø)	⬆️

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