[http] Use POST method only if the form parameter list is non-empty

[http] Use POST method only if the form parameter list is non-empty

An attempt to use an existent but empty form parameter list will
currently result in an invalid POST request since the Content-Length
header will be missing.

Fix by using GET instead of POST if the form parameter list is empty.
This is a non-breaking change (since the current behaviour produces an
invalid request), and simplifies the imminent generalisation of the
parameter list concept to handle both header and form parameters.

Signed-off-by: Michael Brown <mcb30@ipxe.org>