fix: request overrider checks req.headers to parse body as json

[ArTiSTiX]

fixes #1156

Request overrider was using headers from options.
But some library don't set headers via options.

Use req.headers (retreived before in the same function) to ensure body is parse as JSON when header Content-Type=application/json

[gr2m]

Thanks @ArTiSTiX for the pull request. Could you please add a test?

[ArTiSTiX]

@gr2m done.

I updated the old test (which was giving a false positive due to usage of request) by using node's http package and setHeader to mutate request (the approach failing, used by some other packages like superagent).

I added a test to be sure requestBody is not parsed when Content Type is not application/json.

[gr2m]

looks good to me, thanks!

[lock]

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