Response data is always string encoded can't send a binary body

[zootalures]

Description

Steps to reproduce the issue:

1. Create a function that returns a bytes body

import io
import json
import logging
from PIL import Image, ImageDraw
from fdk import response
def handler(ctx, data: io.BytesIO=None):
    img = Image.new('RGB', (60, 30), color='red')
    d = ImageDraw.Draw(img)
    d.text((10, 10), "hello world", fill=(255, 255, 0))
    # write png image to memory 
    output =  io.BytesIO()
    img.save(output, format="PNG")
    # get the bytes of the image 
    imgbytes = output.getvalue()
    return response.Response(
        ctx, response_data=imgbytes,
        headers={"Content-Type": "image/png"}
    )

Describe the results you received:
A string encoded bytes image

(venv) (base) ➜  fdk-python git:(oc/fixBinaryResponses) ✗ curl -v  -X POST http://localhost:8080/invoke/01EC0074S7NG8G00GZJ0000001
*   Trying ::1...
* TCP_NODELAY set
* Connected to localhost (::1) port 8080 (#0)
> POST /invoke/01EC0074S7NG8G00GZJ0000001 HTTP/1.1
> Host: localhost:8080
> User-Agent: curl/7.54.0
> Accept: */*
>
< HTTP/1.1 200 OK
< Content-Length: 614
< Content-Type: image/png
< Fn-Call-Id: 01EC7NB58GNG8G00GZJ000000D
< Fn-Fdk-Version: fdk-python/dev3
< Date: Thu, 02 Jul 2020 11:42:14 GMT
<
* Connection #0 to host localhost left intact
b’\x89PNG\r\n\x1a\n\x00\x00\x00\rIHDR\x00\x00\x00<\x00\x00\x00\x1e\x08\x02\x00\x00\x00\xff\xfa\xea\x18\x00\x00\x00\xa9IDATx\x9c\xed\x96\xd1\n\x80 \x0cE\xb7\xe8\xff\x7f\xd9\x1e”Y\xadt\x89\xde\x12\xeey\x880e\xa7\xeb(5\xc9|,_\x0b\xb4@i\x14\x94FAi\x14\x94FAi\x14k\xe5\xf9~4\xd1\xc0\x1c13\xfdHWz$\xad\xce\xcc\x8ft\xa5\x96\xf4\xce%\xef\x1c\xe4+\xb3\xf2\xaa\\“\x1dW17r\xda\xc3X\xd2v\xd3\x93\x1b\x89\x1b\x17V\xf97\xd1\xbbU*\xd2\xde\x1e\x83\x8e\xe16\x97gb\xed\xe1\x19\xd1\xb2\x1a\xcdB+\xd3.\xbdu\xfbq\x88T\xf2\xdd\xf9T.\xb7\xb5\x158\x97\xaeI\xff\x92)\x7f.\x94FAi\x14\x94FAi\x14SJo\x89l”=T\n?L\x00\x00\x00\x00IEND\xaeB`\x82'%

Describe the results you expected:
A binary image