#64 Fix response Content-Type header.

[yukimochi]

Summary

Fix #64 . Response message should has header {"Content-Type":"application/json"}, if it is message object. Now always message is sent with {"Content-Type":"text/html"}, so slack server treat them text message. Thus, when response message object, show serialized json string in screen.

Requirements (place an x in each [ ])

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

[codecov[bot]]

Codecov Report

Merging #65 into master will increase coverage by <.01%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #65      +/-   ##
==========================================
+ Coverage   99.08%   99.09%   +<.01%     
==========================================
  Files           3        3              
  Lines         219      220       +1     
==========================================
+ Hits          217      218       +1     
  Misses          2        2

Impacted Files	Coverage Δ
src/http-handler.js	98.36% <100%> (+0.02%)	:arrow_up:

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[aoberoi]

this looks great! thank you for catching this problem and providing a solution. my only concern is that our tests should verify this behavior, too. seeing as how the existing tests passed with acceptable coverage, i will merge as-is. i'll open another PR to make the tests more complete.