Remove ensureHeaderWritten() from writeNode()
This updates documentation of AbstractNormalizedNodeDataOutput
to make it clear that the NormalizedNodeStreamWriter aspect of
it is an implementation detail and that the stream has been
header-initialized by the time any of those methods are called.
The clarification renders calling ensureHeaderWritten() from
writeNode() superfluous, as at that point it is guaranteed to
be a no-op.
Change-Id: I85a5c1304849fe5bc737fa51bd4b40ff2dd8c08c
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>