Incorrect Content-Length header

The Content-Length header is calculated incorrectly causing the express server to return a 400 Bad Request response which crashes the node-jsonstore when it tries to JSON.parse that response. For me I've been able to use the node-jsonstore post method sometimes, but it seams when I try to save some text using editor.js it will not save to the jsonstore database. My guess is that some characters included in that output take more than 1 byte to store and therefore the string.length value given the the Content-Length header is no longer correct and makes express return a 400. A fix for this would be to replace `stringifiedData.length` with `Buffer.byteLength(stringifiedData)` [(Buffer.byteLength)](https://nodejs.org/api/buffer.html#buffer_class_method_buffer_bytelength_string_encoding). This would need to be done on both the post and put methods.
Here is a StackOverflow answer that suggest using Buffer.byteLength instead:
[What is the correct method for calculating the Content-length header in node.js](https://stackoverflow.com/questions/17922748/what-is-the-correct-method-for-calculating-the-content-length-header-in-node-js)
Here is the line of code where the change needs to happen. The change aswell needs to be done on line 84 [index.js (line 53)](https://github.com/bluzi/node-jsonstore/blob/master/index.js#L53).

I tried copying the index.js file over to my project and importing that instead of 'node-jsonstore'. In that file I commented out the lines that set these headers and my code started working and the stuff saved correctly. I do though think it's better to set the header to a correct value instead of not setting it at all, so using Buffer.byteLength would be better.