BUG/MEDIUM: httpclient: Fix loop consuming HTX blocks from the response channel

BUG/MEDIUM: httpclient: Fix loop consuming HTX blocks from the response channel

When the HTTP client consumes the response, it loops on the HTX message to
copy blocks content and it removes blocks by calling htx_remove_blk(). But
this function removes a block and returns the next one in the HTX
message. The result must be used instead of using htx_get_next(). It is
especially important because the block used in htx_get_next() loop was
removed. It only works because the message is not defragmented during the
loop.

In addition, the loop on the response was simplified to iter on blocks
instead of positions.

This patch must be backported to 2.5.