fix: parse 503 plain text error

[rhajek]

Proposed Changes

This PR fixes handling of plain-text http errors.

Checklist

• CHANGELOG.md updated
• Rebased/mergeable
• A test has been added if appropriate
• rake test completes successfully
• Commit messages are in semantic format
• Sign CLA (if not already signed)

[codecov-commenter]

Codecov Report

Merging #90 (0efef45) into master (c9a3f0a) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #90   +/-   ##
=======================================
  Coverage   98.99%   98.99%           
=======================================
  Files          16       16           
  Lines         793      794    +1     
=======================================
+ Hits          785      786    +1     
  Misses          8        8           

Impacted Files	Coverage Δ
lib/influxdb2/client/influx_error.rb	100.00% <100.00%> (ø)

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[bednar]

LGTM 👍