[guava] `Multimap` deserializer does not honor DeserializationFeature.ACCEPT_SINGLE_VALUE_AS_ARRAY

[cowtowncoder]

(moved from FasterXML/jackson-datatype-guava#89 by @neilmcguigan)

Should be able to have

enable(DeserializationFeature.ACCEPT_SINGLE_VALUE_AS_ARRAY);

class Foo {
    public Multimap<String,String> bar;
}

with JSON

{
  bar: "qux"
}

However, this throws exception:

JsonMappingException: Expecting START_ARRAY, found VALUE_STRING`

Please note that this works:

class Foo {
    List<String> bar;
}

with JSON:

{
  bar: "qux"
}

when

enable(DeserializationFeature.ACCEPT_SINGLE_VALUE_AS_ARRAY);