fix(Server): fix responding with application/json when using custom servers

[chentsulin]

Got the following error when provide a { } object to the res.end() method.

TypeError [ERR_INVALID_ARG_TYPE]: The "chunk" argument must be of type string or an instance of Buffer. Received an instance of Object

Fix it by attaching the Content-Type: application/json; charset=utf-8 header.

[codecov-io]

Codecov Report

Merging #700 into master will decrease coverage by 0.07%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #700      +/-   ##
==========================================
- Coverage    80.9%   80.83%   -0.08%     
==========================================
  Files         108      108              
  Lines        4489     4493       +4     
  Branches     1109     1122      +13     
==========================================
  Hits         3632     3632              
+ Misses        811      810       -1     
- Partials       46       51       +5

Impacted Files	Coverage Δ
packages/bottender/src/server/Server.ts	9.75% <0%> (-0.51%)	⬇️
packages/bottender/src/getClient.ts	55.55% <0%> (ø)	⬆️
packages/create-bottender-app/src/shared/log.ts	0% <0%> (ø)	⬆️

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