Wrong content length header

[herzaso]

Server: Tornado 4.2.1 and sockjs 1.0.2
Client: Javascript, Firefox

I'm trying to establish a realtime connection from a local file (file:///) to a remote server.
The response headers I get from a call to "http://://info?t=1444720615830" are:

Access-Control-Allow-Credentials: true
Access-Control-Allow-Origin: *
Cache-Control: no-store, no-cache, must-revalidate, max-age=0
Content-Length: 87
Content-Type: application/json; charset=UTF-8
Date: Tue, 13 Oct 2015 07:17:08 GMT
Etag: "eef3578f083ee9ef36b811fe004fbd996ac00192"
Server: TornadoServer/4.2.1

while in the response body I get "SyntaxError: JSON.parse: unexpected end of data at line 1 column 1 of the JSON data"

I Firefox developer tools, there is a button to "Edit and Resend" a request and when I do it (resend without editing" I get a different response headers:

Access-Control-Allow-Credentials: true
Access-Control-Allow-Origin: *
Cache-Control: no-store, no-cache, must-revalidate, max-age=0
Content-Length: 85
Content-Type: application/json; charset=UTF-8
Date: Tue, 13 Oct 2015 07:21:52 GMT
Etag: "529027274ad34b6e2600bbfb60e10e7322506ea0"
Server: TornadoServer/4.2.1

(I marked the differences in bold)

The response body of the resent request is now a valid JSON object
{origins: [':'], websocket: true, cookie_needed: true, entropy: 16239493526095616}

Am I missing something or is there a problem with the content length?

[herzaso]

It appears that this issue is that same as issue #177
After overriding the preflight with Access-Control-Allow-Origin=null, it works (no relation to the content length)...