Remove trailing hyphen from auto heading ID

[jmooring]

Applicable when autoHeadingIDType is either github or github-ascii.

When autoHeadingIDType is blackfriday, the existing code removes
trailing whitespace while iterating through the characters, using
a boolean "futureDash" mechanism.

Fixes #6798

[bep]

Thanks for this,

These "auto ID" schemes are very "compatibility aware" so to speak, so I'm not keen to change stuff to make it prettier -- but I would be happyt to merge this if the "blackfriday ID" behaved differently before we got this Goldmark upgrade, did it?

[jmooring]

Without this PR, and with config.toml as follows:

[markup.goldmark.parser]
autoHeadingIDType = "blackfriday"

trailing spaces are removed from the auto heading ID.

I did not test < 0.60.

[jmooring]

Just tested with Hugo Static Site Generator v0.59.1-D5DAB232/extended linux/amd64 BuildDate: 2019-10-31T15:28:09Z.

Example markdown: # Foo #

Parser	Version	HTML
Blackfriday	0.59.1	<h1 id="foo">Foo</h1>
Blackfriday	0.73	<h1 id="foo">Foo</h1>
Goldmark with autoHeadingIDType = blackfriday	0.73	<h1 id="foo">Foo</h1>
Goldmark with autoHeadingIDType = github	0.73	<h1 id="foo-">Foo</h1>	<--
Goldmark with autoHeadingIDType = github-ascii	0.73	<h1 id="foo-">Foo</h1>	<--
Blackfriday	0.73 + this PR	<h1 id="foo">Foo</h1>
Goldmark with autoHeadingIDType = blackfriday	0.73 + this PR	<h1 id="foo">Foo</h1>
Goldmark with autoHeadingIDType = github	0.73 + this PR	<h1 id="foo">Foo</h1>
Goldmark with autoHeadingIDType = github-ascii	0.73 + this PR	<h1 id="foo">Foo</h1>

[bep]

Note that the relevant thing to look at in the above is how GitHub behaves -- but I added this test case to https://github.com/bep/portable-hugo-links/blob/master/blog/p2.md#trailing-space -- which shows that Hugo/GitHub differs.

[github-actions]

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