Fix exec command parameter expansion

[mbohlool]

Simplify command parameter expansion of websocket_call that will also fix a bug with it (kubernetes-client/python#346 originally tried to fix this before we move websocket code to the base repo). cc @eyeofthefrog

[codecov-io]

Codecov Report

Merging #35 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #35   +/-   ##
=======================================
  Coverage   93.36%   93.36%           
=======================================
  Files          11       11           
  Lines         829      829           
=======================================
  Hits          774      774           
  Misses         55       55

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[dims]

LGTM 👍 we could add a test case when possible

[mbohlool]

Thanks @dims. I spend a little time on writing a test but I needed to mock WebSocket class and I didn't find an easy way to do it. I will merge this without a test for now.