by default, libcurl converts POST requests to GET upon redirect. this does not match RFC, but most browsers do it that way.
options to control this conversion should be exposed.

from http://curl.haxx.se/libcurl/c/curl_easy_setopt.html#CURLOPTPOSTREDIR

Pass a bitmask to control how libcurl acts on redirects after POSTs that get a 301, 302 or 303 response back. A parameter with bit 0 set (value CURL_REDIR_POST_301) tells the library to respect RFC 2616/10.3.2 and not convert POST requests into GET requests when following a 301 redirection. Setting bit 1 (value CURL_REDIR_POST_302) makes libcurl maintain the request method after a 302 redirect whilst setting bit 2 (value CURL_REDIR_POST_303) makes libcurl maintain the request method after a 303 redirect. The value CURL_REDIR_POST_ALL is a convenience define that sets all three bits.

The non-RFC behaviour is ubiquitous in web browsers, so the library does the conversion by default to maintain consistency. However, a server may require a POST to remain a POST after such a redirection.