stop redirecting POSTs

LWP::UserAgent does not support redirecting a POST as a POST.  The
requests_redirectable option will follow a redirect result for a POST
but as a GET.  This appears to be the desired behavior according to RFC
2616 10.3.3.  Rather than implementing a POST redirect we will just
throw an error that suggests the server URL may not be correct.
Merge pull request #36 from nnutter/post

stop redirecting POSTs