luci.http.protocol: Allow posts to contain more than a single chunk

-- This is a backport of https://github.com/openwrt/luci/pull/258 for the BB branch.  I know it's not strictly necessary for BB, but I can't work out an easy way of carrying this as a local patch.

The ltn12.pump.step() loop in parse_message_body was incorrectly testing
for EOF, and returning after the first chunk, even if further chunks
were available.  The loop in parse_message_header made the correct
check, and test code making raw ajax json posts now succeeds for posts
>= 2048 bytes.

Signed-off-by: Karl Palsson <karlp@remake.is>