Help decoding JSON data from socket.http.request

[mooselight]

Dear All,
I'm trying to retrieve consumption info from an "Ecocompteur" box from Legrand with the following code, but it does not work as expected. 
The returned Content-Type is "text/plain" while it should be "application/json", so the json.decode function does not work.

Is there another way to retrieve data values from the returned "json" data ?
Thanks in advance

Code:

Accessing "http://192.168.1.xx/inst.json" give this data in return :

{
   "data1":27.000000,
   "data2":0.000000,
   "data3":0.000000,
   "data4":0.000000,
   "data5":0.000000,
   "data6":0.000000,
   "data6m3":0.000000,
   "data7":0.000000,
   "data7m3":0.000000,
   "heure":22,
   "minute":5
}

Code:

require 'ltn12'
require 'socket.http'
json = require("json")

local request_url = "http://192.168.1.xx/inst.json"
local response_body = {}

local body, code, hdrs, stat = socket.http.request
  {
    url = request_url;
    sink = ltn12.sink.table(response_body);
  }

log (body, code, hdrs, stat)

-- local response_decode = json.decode(table.concat (response_body))

log (response_body)

-- local data1 = response_decode.data1
-- local data2 = response_decode.data2
...

Here is the log output :

Code:

Ecocompteur 01.11.2017 20:33:54
* arg: 1
 * number: 1
* arg: 2
 * number: 200
* arg: 3
 * table:
  [server]
   * string: lwip/3.0
  [content-type]
   * string: text/plain
* arg: 4
 * string: HTTP/1.0 200 OK


Ecocompteur 01.11.2017 20:33:54
* string: {
   "data1":35.000000,
   "data2":0.000000,
   "data3":1.000000,
   "data4":0.000000,
   "data5":0.000000,
   "data6":0.000000,
   "data6m3":0.000000,
   "data7":0.000000,
   "data7m3":0.000000,
   "heure":20,
   "minute":33
}HTTP/1.0 200 OK
Server: lwip/3.0
Content-Type: text/plain

{
   "data1":35.000000,
   "data2":0.000000,
   "data3":1.000000,
   "data4":0.000000,
   "data5":0.000000,
   "data6":0.000000,
   "data6m3":0.000000,
   "data7":0.000000,
   "data7m3":0.000000,
   "heure":20,
   "minute":33
}