Fix request body for GET and HEAD methods

[grongor]

There is no limitation in cURL that would prevent these methods to have a body. And without the body for these requests you are unable to use APIs that require it, such as https://docs.confluent.io/current/kafka-rest/api.html#get--consumers-(string-group_name)-instances-(string-instance)-offsets .

[Nyholm]

I know that the intention is correct. The GET and HEAD requests can have bodies. Im not sure about the implementation because I dont know this client well enough.

Can @mekras have a look?