Adding custom header in order to provide a POST with custom MIME types

Our server desires a oAuth-request with a POST MIME type of 'application/x-www-form-urlencoded'. In order to get it work with the OAuth2Client we needed to add the support of creating the desired POST request.

The OAuth2Client in this state will only work with application/x-www-form-urlencoded and the 'multipart/form-data'. More MIME types must be added if needed. But IMO this is a good start to do so.

If the (new) kNXOAuth2AccountStoreConfigurationCustomHeaderFields key is set in the NXOAuth2AccountStore configuration the all header fields of the resulting HTTP-Request are replaced with the value for this key.
This actually adds a warning when compiled in 32bit. Either leave the cast to `int` in or use `"%zd"` which adapts to the native architecture word width.