Allow custom headers in request

[fabsrc]

Currently it is not possible to set custom headers when requesting the keys.

In our case this lead to the following problem: the "request" module sets the Accept header to application/json when json: true is used and no Accept header is set explicitly. See:

request({ json: true, uri: this.options.jwksUri, strictSSL: this.options.strictSsl }, (err, res) => {

https://github.com/request/request/blob/a92e138d897d78ce19dd70bd1ad271c7f9c6a23d/request.js#L1279-L1281

However if the server that stores the JWKS is configured to return a application/jwk-set+json content-type, a request with Accept: application/json is declined with a 406 error. To fix this we need to set the Accept header to either application/jwk-set+json or *.

It would be great if custom headers could be configured in the options.