Fix serializing generic enums

This fixes generic enums with variants that don't use all lifetime
and typarams. Closes #88.

serde_tests/tests/test_macros.rs

@@ -123,6 +123,14 @@ enum DeEnum<B, C: /* Trait */, D> /* where D: Trait */ {
     },
 }
 
+#[derive(Serialize)]
+enum Lifetimes<'a> {
+    LifetimeSeq(&'a i32),
+    NoLifetimeSeq(i32),
+    LifetimeMap { a: &'a i32 },
+    NoLifetimeMap { a: i32 },
+}
+
 #[test]
 fn test_named_unit() {
     let named_unit = NamedUnit;
@@ -438,3 +446,32 @@ fn test_de_enum_map() {
         }
     );
 }
+
+#[test]
+fn test_lifetimes() {
+    let value = 5;
+    let lifetime = Lifetimes::LifetimeSeq(&value);
+    assert_eq!(
+        json::to_string(&lifetime).unwrap(),
+        "{\"LifetimeSeq\":[5]}"
+    );
+
+    let lifetime = Lifetimes::NoLifetimeSeq(5);
+    assert_eq!(
+        json::to_string(&lifetime).unwrap(),
+        "{\"NoLifetimeSeq\":[5]}"
+    );
+
+    let value = 5;
+    let lifetime = Lifetimes::LifetimeMap { a: &value };
+    assert_eq!(
+        json::to_string(&lifetime).unwrap(),
+        "{\"LifetimeMap\":{\"a\":5}}"
+    );
+
+    let lifetime = Lifetimes::NoLifetimeMap { a: 5 };
+    assert_eq!(
+        json::to_string(&lifetime).unwrap(),
+        "{\"NoLifetimeMap\":{\"a\":5}}"
+    );
+}