Fix enum deserialization from env values

Fixes #31

tests/env.rs

@@ -0,0 +1,19 @@
+use serde::Deserialize;
+use confique::{Config};
+
+#[derive(Debug, Deserialize)]
+enum Foo { A, B, C }
+
+
+#[test]
+fn enum_env() {
+    #[derive(Config)]
+    struct Conf {
+        #[config(env = "FOO")]
+        foo: Foo,
+    }
+
+    std::env::set_var("FOO", "B");
+    let conf = Conf::builder().env().load();
+    assert!(matches!(conf, Ok(Conf { foo: Foo::B })));
+}