Setting any header throws Preflight keepalive error in Chromium

[carcinocron]

What is the current behavior?

When using the config:

headers: {
          'Content-Type': 'application/json',
},

There results the error:

Preflight request for request with keepalive specified is currently not supported

It works without the configuration.

What is the expected behavior?

It should add the header and continue or throw a less cryptic error.

Which versions of Raven.js, and which browser and OS are affected by this issue? Did this work in previous versions of Raven.js? Are you using the CDN (http://ravenjs.com)? Are you using hosted Sentry or on-premises? If on-premises, which version (e.g. 8.7.0)?

raven-js@^3.21.0:
  version "3.26.4"
  resolved "https://registry.yarnpkg.com/raven-js/-/raven-js-3.26.4.tgz#32aae3a63a9314467a453c94c89a364ea43707be"
 
raven@^2.3.0:
  version "2.6.3"
  resolved "https://registry.yarnpkg.com/raven/-/raven-2.6.3.tgz#207475a12809277ef54eaceafe2597ff65262ab4"
  dependencies:
    cookie "0.3.1"
    md5 "^2.2.1"
    stack-trace "0.0.10"
    timed-out "4.0.1"
    uuid "3.0.0"

Chromium:

Version 68.0.3440.75 (Official Build) Built on Ubuntu , running on Ubuntu 16.04 (64-bit)