Allow providing custom HTTP headers to the endpoints

When the connection library is setting a default Accept header which doesn't match the requirements of the authentication server or sending an invalid Accept header which is rejected by the authentication server, the authentication fails.

This could be solved by explicitly setting a Accept header that matches the requirements of the authentication server. Therefore, the oidc extension got the method

setAccept(final String accept)

https://bitbucket.org/connect2id/oauth-2.0-sdk-with-openid-connect-extensions/src/ae04c503efb5ee5695bbc3d42b988e7bf0670ef0/src/main/java/com/nimbusds/oauth2/sdk/http/HTTPRequest.java#lines-372

An example would be the the Microsoft Graph which checks for a correct Accept header and the unresolved OpenJDK issue, that it's sending an invalid Accept header if none is set explicitly.

https://bugs.openjdk.java.net/browse/JDK-8163921

This case also have been discussed https://forum.xwiki.org/t/issues-with-openid-office365/3548/17 without a solution yet. I hope, this issue helps clarifying the situation and the need of this feature to avoid faulty or incompatible default values.