JSON requests support

I'm trying to process POST requests with JSON body. For now I don't see a way how to do it.
`parsepostdata` doesn't recognize the header `Content-Type: application/json` and interprets the data as parameters of a query only.

Any suggestions why it is not implemented? Should I use other libraries for JSON processing like Mux.jl or is it just non implemented feature?

```HTTP
POST /document HTTP/1.1
Host: localhost
Content-Type: application/json
cache-control: no-cache
Postman-Token: 6761fa70-a0d8-413a-a31c-79e882cc17d2

{
"key":"document",
"value":{"text":"some text"},
"description":"",
"type":"text",
"enabled":true
}------WebKitFormBoundary7MA4YWxkTrZu0gW--
```