SparkPost cc and bcc improvements

[caronc]

Description:

Related issue (if applicable): #307

Just some added testing and overlooked the correct handling of cc/bcc emails.

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.
• No lint errors (use flake8)
• 100% test coverage

[codecov-commenter]

Codecov Report

Merging #309 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #309   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           84        84           
  Lines        10483     10483           
  Branches      1750      1749    -1     
=========================================
  Hits         10483     10483           

Impacted Files	Coverage Δ
apprise/plugins/NotifySparkPost.py	100.00% <100.00%> (ø)

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