Hub#capture_message should check its argument's type

[st0012]

Fixes #1574

[codecov-commenter]

Codecov Report

Merging #1577 (bde6c22) into master (65a51e0) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head bde6c22 differs from pull request most recent head cd74404. Consider uploading reports for the commit cd74404 to get more accurate results

@@           Coverage Diff           @@
##           master    #1577   +/-   ##
=======================================
  Coverage   98.55%   98.55%           
=======================================
  Files         127      127           
  Lines        7001     7005    +4     
=======================================
+ Hits         6900     6904    +4     
  Misses        101      101           

Impacted Files	Coverage Δ
sentry-ruby/lib/sentry/hub.rb	100.00% <100.00%> (ø)
sentry-ruby/spec/sentry/hub_spec.rb	100.00% <100.00%> (ø)

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