Add binary body representation for http(s) requests for /imposters/{key} endpoint

[tbarabanov]

Expected behaviour

httpRequest.js transformed.body is a string that is not what I expect to see for instance in case of application/octet-stream content.
I would like to have a binary representation of the recorded request body as well as string.

{
"ip":...,
"body": "...",
"bytes": ""iVBORw0KGgoAAAANSUhEUgAABjIAAAYCCAYAAACR..." // body as bytes (base64 string)
}

Actual behaviour

At the moment requests history returns body as string only.

Software versions used

OS         :
mountebank : 2.7.0
node.js    :
  (only if installed via npm)
Installation method :
  (npm, zip, tar, pkg, deb, rpm)