Add support for bytes when binary_types accept application/json

[dbritto-dev]

import json
import gzip
from chalice import Chalice, Response

app = Chalice(app_name='gzipcontent')
app.api.binary_types.append('application/json')

@app.route('/')
def index():
    blob = json.dumps({'hello': 'world'}).encode('utf-8')
    payload = gzip.compress(blob)
    return Response(body=payload, headers={'Content-Type': 'application/json', 'Content-Encoding': 'gzip'})

Fixes #988, when binary_types is application/json and you pass bytes in the body the code throws error

Description of changes:
This patch check if the body is a string and apply .encode('utf-8') if not, let me forward the bytes from gzip.compress. Without this patch the code above doesn't work.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-io]

Codecov Report

Merging #986 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #986   +/-   ##
=======================================
  Coverage   95.52%   95.52%           
=======================================
  Files          27       27           
  Lines        4449     4449           
  Branches      558      558           
=======================================
  Hits         4250     4250           
  Misses        128      128           
  Partials       71       71

Impacted Files	Coverage Δ
chalice/app.py	96.75% <100%> (ø)	⬆️

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[joguSD]

I've continued this PR by squashing and rebasing against the latest master in #1013

[dbritto-dev]

Thank you @joguSD