Set type on enum schemas (#28)

schemars_derive/src/schema_exprs.rs

@@ -89,6 +89,7 @@ fn expr_for_external_tagged_enum<'a>(
 
     let unit_names = unit_variants.iter().map(|v| v.name());
     let unit_schema = schema_object(quote! {
+        instance_type: Some(schemars::schema::InstanceType::String.into()),
         enum_values: Some(vec![#(#unit_names.into()),*]),
     });