Fix bug where a code block at the start of a file was not recognised.
I was actually adding this test to check that the `source_mode_unset`
mechanism in `doc_info_grammar.cpp` was working, but it ended exposing
this bug.
[SVN r85590]
In quickbook 1.6, nested documents default to C++, regardless of the
parent's source mode so that they are interpreted as if they are
standalone documents.
[SVN r75528]