Tests for combination of + operator, percent-encoded value, and prefix modifier are missing

[lo48576]

Consider expanding {+abcdef:5} with abcdef := "%61%62%63%64%65%66".
(And in similar way, {+invalid_utf8:2} with invalid_utf8 := %FF%FF%FF%FF.)
The spec (RFC 6570) is not so clear in what the result should be, and various implementations would produce different results, so this kind of inputs should be included to the test suite.

[lo48576]

By the way, I'm not sure but I think the expected result would be %61%62%63%64%65 (and %FF%FF).

Note that this numbering is in characters, not octets, in order to avoid splitting between the octets of a multi-octet-encoded character or within a pct-encoded triplet.

-- https://www.rfc-editor.org/rfc/rfc6570.html#section-2.4.1