Fixing Too many properties to enumerate on big XML documents

[tryggingamidstodin]

This PR fixes an error when receiving a large XML document. When checking for the body length Object.keys is used causing the "Too many properties to enumerate" error for large XML documents.

When checking a length of buffer (when using raw body parser) you get the same results of

const buffer = Buffer.from('abc')
console.log(buffer.length) // 3

const buffer = Buffer.from('abc')
const keys = Object.keys(buffer)
console.log(keys.length) //3

When checking a length of a text (when using text body parser) you get the same results of

const txt = 'abc'
console.log(text.length) // 3

const text = 'abc'
const keys = Object.keys(buffer)
console.log(keys.length) //3

Except you won't reach the property limit of 65535](https://stackoverflow.com/a/9283945/5189472) for large documents. This PR solves that issue.

[coveralls]

Coverage remained the same at 95.068% when pulling 080cf06 on tryggingamidstodin:master into 95fa71f on vpulim:master.