HTML API: Ensure attribute value prefixes match entire search string

[sirreal]

Ensure that a shorter haystack does not match a longer prefix when searching for prefix matches:

var_dump( WP_HTML_Decoder::attribute_starts_with( 'm', 'matchme' ) );
// bool(true)
var_dump( WP_HTML_Decoder::attribute_starts_with( '', 'empty always matches' ) );
// bool(true)

Trac ticket: https://core.trac.wordpress.org/ticket/65372

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[dmsnell]

this needs an illustrative comment showing when and why we are using min()

the fj/f example is a good one. attribute value contains fj, search prefix is f, decoded is fj

[dmsnell]

would this logic be clearer and still held if, instead of adjusting numbers here, that we cut the $next_chunk short if if the prefix length plus its own length is beyond the search/needle length?

that is, if we have fi as the replacement, but the search length is one less than that, we would simply $next_chunk = $length_okay ? $next_chunk : substr( $next_chunk, 0, $max_length )?