Add key-value pairs found in the attributes list of Header.Attr as
XML attributes on the corresponding section element.
Any key name not allowed as an XML attribute name is dropped, as
are keys with invalid values where they are defined as enums in
DocBook, and xml:id (for DocBook 5)/id (for DocBook 4) to not
intervene with computed identifiers.
Docbook reader produces a `Div` with `title` class for `<title>` element
within an “admonition” element. Markdown writer then turns this
into a fenced div with `title` class attribute. Since fenced divs
are block elements, their content is recognized as a paragraph
by the Markdown reader. This is an issue for Docbook writer because
it would produce an invalid DocBook document from such AST –
the `<title>` element can only contain “inline” elements.
Let’s handle this invalid special case separately by unwrapping
the paragraph before creating the `<title>` element.
* XML.toEntities: changed type to Text -> Text.
* Shared.tabFilter -- fixed so it strips out CRs as before.
* Modified writers to take Text.
* Updated tests, benchmarks, trypandoc.
[API change]
Closes#3731.
