oauthex: fix content type check in getJSON

[nikolavp]

The content type header contains full mimetype string which might contain parameters. The getJSON function doesn't account for this and so some responses fail to parse although they are perfectly acceptable. For example

curl -vv 'https://github.com/.well-known/oauth-authorization-server/login/oauth'

returns a content type header with the value

content-type: application/json; charset=utf-8

which is perfectly valid but fails in the current logic.

[jba]

Thanks!