Contradicting default/implied value for omitted ACTION column

[khaeru]

This overlaps a little with #29 (comment).

In the latest release, the section ## Column content (all rows after header) includes:

sdmx-csv/data-message/docs/sdmx-csv-field-guide.md

Line 67 in 9a65b13

- For convenience, if this column is absent then the *Merge* action is assumed.

and mentions that "A" and "I" are deprecated.

Earlier at ## Columns headers (first row), a different default is given, using a value that is marked as deprecated.

sdmx-csv/data-message/docs/sdmx-csv-field-guide.md

Line 46 in 9a65b13

- The header field of the next column should contain the term `ACTION`. For convenience, if this column is not present, a default action ("Information") is assumed for the whole message.