Move fix for #1114 to Truncator::truncateLetters

[dliessi]

The original fix provided by #1125 in Utils::truncateHTML compared the
summary size with the full HTML string length.
Thus, the string was still being truncated (and the HTML rewritten) even
when only the HTML string length, and not the text length, exceeded the
summary size.

[dliessi]

The failing test is

$this->assertEquals('<input type="file" id="file" multiple>', Utils::truncateHtml('<input type="file" id="file" multiple />', 6));

I suspect that the first argument of assertEquals was chosen to be different from the first argument of Utils::truncateHtml only because Truncator::innerHtml rewrites the HTML.
Is this true? Can I make them equal? Which form should be retained (with or without the closing /)?

[dliessi]

I chose to keep the XHTML syntax, based on other code generated by Grav, although in HTML it is unnecessary.

[rhukster]

looks good.. cheers.