send POST request data in body instead of query params to avoid HTTP 414 errors

[debreczeni]

avoid receiving 414 Request-URI Too Large and Oj::ParseErrors in turn

[codecov-commenter]

Codecov Report

Merging #41 into master will decrease coverage by 3.22%.
The diff coverage is 50.00%.

@@             Coverage Diff             @@
##            master      #41      +/-   ##
===========================================
- Coverage   100.00%   96.77%   -3.23%     
===========================================
  Files            4        4              
  Lines           30       31       +1     
===========================================
  Hits            30       30              
- Misses           0        1       +1     

Impacted Files	Coverage Δ
lib/pushover/message.rb	83.33% <50.00%> (-16.67%)	⬇️

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