RST writer: tweaks to header normalization.

These changes are intended to make the writer more
useful to people who are processing small fragments,
which may for example look like this:

    ### third level header from previous section

    ## second level header

Previously such fragments got turned into two
headers of the same level.  The new algorithm
avoids doing any normalization until we hit the
minimal-level header in the fragment (here, the
second level header).

Closes #2394.

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