Use better default Accept-header for JDKHttpClient

[danielFesenmeyer]

Currently the JDKHttpClient uses the following Accept header, if none is specified by the calling code: text/html, image/gif, image/jpeg, *; q=.2, */*; q=.2

As described in a JDK issue [1], this causes problems with several server side libraries.

I had problems to get the device authorization flow working with Keycloak due to this issue.

What worked for me was extending OAuth20Service, and overriding the execute methods, and patching the request like that:
request.addHeader("Accept", "*/*");

The accept header */* is used per default by Postman and curl, so it seems to be a meaningful default - at least better than html, image and so on (afaik, scribejava is mostly dealing with JSON).

I would suggest to change method JDKHttpClient#prepareConnectionForBodyAndGetOutputStream to set this meaningful default Accept header, if none has been already set. [2]

[1] https://bugs.openjdk.org/browse/JDK-8163921
[2]

scribejava/scribejava-core/src/main/java/com/github/scribejava/core/httpclient/jdk/JDKHttpClient.java

Line 224 in 49cf8a6

private static OutputStream prepareConnectionForBodyAndGetOutputStream(HttpURLConnection connection,