does_text_contain_section should match preleading ":" in textlinks

When a section header contains a text link with preleading colon,
it should match it too:

== [[This is a section header]] ==

== [[:This is also a valid section header]] ==

Additional comments see doc string.

Change-Id: Ie52c3956b4d82bb71c2113cbff476c3dec61fa0c
Merge "does_text_contain_section should match preleading ":" in textlinks"