fix(electron-auto-updater) Checking for updates from github was failing

[badams]

Appears that github are now more strict about the Accept header, and invalid requests are resulting in a 406 response

I've adjusted the httpExecutor.request method to allow for custom header objects to be passed, so user can specify header (eg. Accept: application/json)

I've also updated the way we retrieve the latest version tag;
when sending Accept: application/json to /:user/:repo/releases/latest github will respond with a JSON payload like below

{
  "tag_name": "v1.0.0",
  ...
}

So it can be easily extracted without relying on redirect headers or the official API

#1038 #1058

[mention-bot]

@badams, thanks for your PR! By analyzing the history of the files in this pull request, we identified @maczikasz to be a potential reviewer.

[develar]

Amazing! Thanks a lot. For some technical (unrelated) reasons PR cannot be accepted right now, but I hope to merge it today.

[develar]

Don't worry about merge conflicts :)

[develar]

[email protected] (tag: next) published with your fix. Tag next because I don't have time to make all tests and we don't yet have special electron tests.