Fix Global Rpc Kill et al failures

Fix Global Rpc Kill et al failures

Global Rpc (Kill|Freeze|Isolate) have been failing for quite some time
(in Neon and Fluorine) with a 500 response when executing RPC
 /restconf/operations/basic-rpc-test:basic-global. It's a no-input RPC
and the robot code passes an empty strng for the content-type and accept
parans to Post_To_Uri. In comparing with another RPC that has no input,
that code passes the ${HEADERS_XML} and ${ACCEPT_XML} constants so I did
the same in the Glocal Rpc tests. It seems something changed in Fluorine
where passng empty content-type/accept headers no longer works.

Change-Id: Iaa4e9ef9dddde8291d1cae7d09f878be0b781ae0
Signed-off-by: Tom Pantelis <tompantelis@gmail.com>