Strip token string parameters of whitespace

[TheFrozenFire]

Summary

If tokens contain whitespace, API calls fail in a confusing manner. See #598.

This change simply strips whitespace from tokens, to avoid confusing failures.

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[claassistantio]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #599 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #599   +/-   ##
=======================================
  Coverage   69.87%   69.87%           
=======================================
  Files          15       15           
  Lines        1726     1726           
  Branches       98       98           
=======================================
  Hits         1206     1206           
  Misses        493      493           
  Partials       27       27

Impacted Files	Coverage Δ
slack/web/base_client.py	78.5% <100%> (ø)	⬆️
slack/rtm/client.py	83.15% <100%> (ø)	⬆️

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