[Bug] SDK emit content_length in http response with string type instead of number.

Xray SDK should send `content_length` with number type in the json segment for the [http request data](https://docs.aws.amazon.com/xray/latest/devguide/xray-api-segmentdocuments.html).

But I get this data when I use the latest node.js sdk version: [data.txt](https://github.com/aws/aws-xray-sdk-node/files/6065846/data.txt)

This is a bug and when our customers try to use OT collector to take place of Xray-daemon, they can't get the data they want.