Test to cover "Value after document-start" (on first line)

[am11]

Consider the following valid cases:

--- blah

--- hello=world

--- +190:20:30

--- x:y

vs. invalid case of mapping after the DocumentStart:

--- x: y

Currently, spec test 27NA and 6LVF etc. cover the cases when document-start followed by value token is on lines, other than the first one.

In the spirit of tightening the net, it would probably be a good idea to add tests clarifying what is allowed after document start marker, and if document-start on first line matters.