fn:element-to-map: restore content-key collision handling

[GuntherRademacher]

QT4 test element-to-map-227 for fn:element-to-map failed:

element-to-map(parse-xml('<a id="zz">babel</a>')/a, {'content-key': '@id'})
Result: { "a": { "@id": array { ("zz", "babel") } } }
Expect: { "a": { "@id": "zz", "#@id": "babel" } }

Commit f56348b applied the general array-based handling of duplicate map keys to collisions involving content-key. However, the specification defines a specific rule for such collisions: # characters are prepended to the requested content key until it is unique. This change restores the previous collision handling for content-key while retaining array-based handling for other duplicate-key cases.