Fix PATCH request issue with top-level container as target

Fix PATCH request issue with top-level container as target

PATCH request to the data root with a top-level container as
target does not work because JsonPatchBodyReader and
XmlPatchBodyReader expect a parent statement in the stack.

Address the issue by setting target schema node to the path
schema context when stack is empty and add test cases for
JsonPatchBodyReader and XmlPatchBodyReader.

JIRA: NETCONF-877
Change-Id: Ibb65ccdf3f03d8a9a9023d75bf288e5e4416ca50
Signed-off-by: Sangwook Ha <sangwook.ha@verizon.com>