UntypedObjectDeserializer mixes multiple unwrapped collections (related to #2733)

[Migwel]

(needed for FasterXML/jackson-dataformat-xml#445)

There's an issue when deserializing XML nodes containing multiple unwrapped collections. For example:

<person>
    <name>a</name>
    <name>b</name>
    <surname>c</surname>
    <surname>d</surname>
</person>

gets deserialized into

{
    name = [ c, d ], 
    surname = d
}

Expected deserialization:

{
    name = [ a, b ], 
    surname = [c, d]
}

[cowtowncoder]

Note: relevant for #2733 that was added in 2.12.0.

[cowtowncoder]

Fixed for 2.12.2.