BUG #15277: ts_headline strips things that look like HTML tags and itcannot be disabled

The following bug has been logged on the website:

Bug reference:      15277
Logged by:          Dan Book
Email address:      grinnz@gmail.com
PostgreSQL version: 9.6.9
Operating system:   CentOS 7
Description:

This post has a good overview of the issue with a reproduction case:
https://stackoverflow.com/questions/40263956/why-is-postgresql-stripping-html-entities-in-ts-headline

I have text that is not HTML and contains things that look like HTML tags.
The headlines are HTML escaped when output. It is very odd to have this text
missing from the resulting headlines and no way to control the behavior.