Don't try to close the aiohttp session if connector_owner is False

[leszekhanusz]

See issue #381

[codecov]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage 👍

Coverage data is based on head (132d4dc) compared to base (f28670d).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #382   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           25        27    +2     
  Lines         2321      2358   +37     
=========================================
+ Hits          2321      2358   +37     

Impacted Files	Coverage Δ
gql/transport/aiohttp.py	100.00% <100.00%> (ø)
gql/utils.py	100.00% <0.00%> (ø)
gql/__init__.py	100.00% <0.00%> (ø)

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[shawnlewis]

LGTM!