rpc: always read response
    
    If the server responds 401 and the response body is empty, the
    client raises ResponseNotReady.  This occurs because:
    
    1. For a non-200 response, the response read only if the
       Content-Length header occurs.
    
    2. The response must be read before another request (e.g. the
       follow-up request with WWW-Authenticate header set), and this
       condition was not met.  For details see
       https://github.com/python/cpython/blob/v3.6.7/Lib/http/client.py#L1305-L1321.
    
    This situation should not arise in regular use, because the client
    either has a session cookie, or, knowing the details of the server
    it is contacting, it establishes the GSS-API context and includes
    the WWW-Authenticate header in the initial request.
    
    Nevertheless, this problem has been observed in the wild.  I do not
    know its ordinary cause(s), but one can force the issue by removing
    an authenticated user's session cache from /run/ipa/ccaches, then
    performing a request.
    
    Resolve the issue by always reading the response.  It is safe to
    call response.read() regardless of whether the Content-Length header
    appears, or whether the body is empty.
    
    Fixes: https://pagure.io/freeipa/issue/7752
    Reviewed-By: Christian Heimes <cheimes@redhat.com>