FIX: (adoc-promote): headings get wrongfully turned into sub-type 2

[fapdash]

If the heading contains whitespace or a newline after the
heading title it gets turned into a sub-type 2 heading.

• add failing tests
• fix issue by throwing away whitespace in match group 3

---

In theory we could change the regex but a comment indicates that some other logic depends on the newline being matched, but I didn't investigate further:

adoc-mode/adoc-mode.el

Lines 689 to 691 in c0906ff

	;; using \n instad $ is important so group 3 is guaranteed to be at least 1
	;; char long (except when at the end of the buffer()). That is important to
	;; to have a place to put the text property adoc-reserved on.