This adds the option to provide the desired character as the separator
for the CSV format (the default remains comma).
A set of characters are excluded though - like CR, LF, `"` - to avoid
slipping onto the CSV-dialects slope. The tab is also forbidden, the
user needs to choose the "tsv" format explicitely.

The TSV format also gets a needed supplemental escaping, that of the '\'
character.

Fix the way the delimiter is being passed around, removing the CSV
member - since its instantiation is a singleton - and pushing it through
function arguments.

The commit fixes a few issues with the "header" URL attribute:
- these was no longer usable, since it wasn't "consumed" by
RestSqlQueryAction before generating the answer (triggering an error
responce on use);
- its value was only checked against "absent", so an incorrect value
(typo like "abaent") could have silently generated an unexpected reply.
- missing

Documentation for the "delimiter" attribute has also been added.

The commit unescapes the deliter param. This is required for the
doc-sourced tests.

Also, implement a switch for param value validation and fix docs.

Co-authored-by: Marios Trivyzas <matriv@gmail.com>

- rip out anything which isn't Delimiter-related.
- undo 'hasHeader' method renaming.

Update the doc to make it clear that the textual CSV, TSV and TXT
formats pass the cursor back to the user through the Cursor HTTP header.