Fix error when deserializing untagged enum

Serde's `ContentDeserializer` and `ContentRefDeserializer`
cannot deserialize struct enum variant associated data when
that data is encoded as a sequence. This failure leads to
errors when decoding an enum nested in another untagged
enum. For example:

    #[derive(Serialize, Deserialize)]
    #[serde(untagged)]
    enum Foo {
        A(Bar),
    }

    #[derive(Serialize, Deserialize)]
    enum Bar {
        B{f1: String},
    }

    let data1 = Foo::A(Bar::B{f1: "Hello".into()});
    let bytes = rmp_serde::to_vec(&data1).unwrap();
    let data2 = rmp_serde::from_slice::<Foo>(&bytes).unwrap();

Deserializing fails with the error `Syntax("data did not
match any variant of untagged enum Foo")`, but the
underlying failure occurs when decoding the associated data
of `Bar::B`.

This pull request fixes the issue by allowing
`ContentDeserializer` and `ContentRefDeserializer` to
deserialize sequence-encoded struct enum variant data.

serde/src/private/de.rs

@@ -1246,6 +1246,9 @@ mod content {
                 Some(Content::Map(v)) => {
                     de::Deserializer::deserialize_any(MapDeserializer::new(v), visitor)
                 }
+                Some(Content::Seq(v)) => {
+                    de::Deserializer::deserialize_any(SeqDeserializer::new(v), visitor)
+                }
                 Some(other) => Err(de::Error::invalid_type(other.unexpected(), &"struct variant"),),
                 _ => Err(de::Error::invalid_type(de::Unexpected::UnitVariant, &"struct variant"),),
             }
@@ -1640,6 +1643,9 @@ mod content {
                 Some(&Content::Map(ref v)) => {
                     de::Deserializer::deserialize_any(MapRefDeserializer::new(v), visitor)
                 }
+                Some(&Content::Seq(ref v)) => {
+                    de::Deserializer::deserialize_any(SeqRefDeserializer::new(v), visitor)
+                }
                 Some(other) => Err(de::Error::invalid_type(other.unexpected(), &"struct variant"),),
                 _ => Err(de::Error::invalid_type(de::Unexpected::UnitVariant, &"struct variant"),),
             }