fix: fix send json when fc use http trigger

[czy88840616]

if we use res.send(json) in http trigger, fc will be throw error.

TypeError: buffer should be a Buffer or a string or a stream.Readable

[gitpod-io]

[codecov-io]

Codecov Report

Merging #155 into master will not change coverage.
The diff coverage is 54.54%.

@@           Coverage Diff           @@
##           master     #155   +/-   ##
=======================================
  Coverage   80.28%   80.28%           
=======================================
  Files          85       85           
  Lines        3181     3181           
  Branches      357      357           
=======================================
  Hits         2554     2554           
  Misses        623      623           
  Partials        4        4           

Impacted Files	Coverage Δ
packages/serverless-fc-starter/src/runtime.ts	85.71% <54.54%> (ø)

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