Unable to serialize top-level Java8 Stream

A `Stream` nested in another object is serialized using a value wrapper just as a collection or array, serializing a `Stream` directly fails when more than one element is available as it tries to write a second root. If the Stream emits only a single element serialization works but without wrapping:
```java
public class StreamTest {
    private static final ObjectMapper OBJECT_MAPPER = new XmlMapper()
            .registerModule(new Jdk8Module());
    
    private static class StreamContainer {
        @JsonProperty
        private Stream<String> stream;
        public StreamContainer(Stream<String> stream) { this.stream = stream; }
    }

    @Test
    public void testTopLevelOneElement() throws JsonProcessingException {
        assertNotEquals("<Head>a</Head>",
                OBJECT_MAPPER.writeValueAsString(Stream.of("a")));
    }
    
    @Test
    public void testTopLevelTwoElements() throws JsonProcessingException {
        try {
            assertEquals("<Head><item>a</item><item>b</item></Head>",
                    OBJECT_MAPPER.writeValueAsString(Stream.of("a", "b")));
        } catch (JsonMappingException e) {
            fail("Trying to output second root?", e);
        }
    }
    
    @Test
    public void testNestedOneElement() throws JsonProcessingException {
        assertEquals("<StreamContainer><stream>a</stream></StreamContainer>",
                OBJECT_MAPPER.writeValueAsString(new StreamContainer(Stream.of("a"))));
    }
    
    
    @Test
    public void testNestedTwoElements() throws JsonProcessingException {
        assertEquals("<StreamContainer><stream>a</stream><stream>b</stream></StreamContainer>",
                OBJECT_MAPPER.writeValueAsString(new StreamContainer(Stream.of("a", "b"))));
    }
}
```

The root element name is `Head` for Stream serialization as it's determined from the actual class name `ReferencePipeline$Head`.

The cause lies in `TypeUtil.isIndexedType(Class<?>)` which just checks for Arrays and Collections and only if this returns true, a field name is written (and _nextName set).

It works for the nested Stream as `_nextName` is set due to the Stream being written as a field (multiple times).

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Unable to serialize top-level Java8 Stream #302

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Unable to serialize top-level Java8 Stream #302

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