net: lib: http_server: Fix trailing headers frame processing

In case client decides to send a trailing headers frame, the last data
frame will not carry END_STREAM flag. In result, with current logic
server would not include END_STREAM flag either, causing the connection
to stall. This commit fixes this logic, so that the server replies
accordingly in case END_STREAM flag is present in the trailing headers
frame.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>