Add test case

lettre_email/tests/build_with_envelope.rs

@@ -0,0 +1,18 @@
+extern crate lettre_email;
+extern crate lettre;
+use lettre_email::EmailBuilder;
+use lettre::{EmailAddress, Envelope};
+
+#[test]
+fn build_with_envelope_test() {
+    let e = Envelope::new(
+        Some(EmailAddress::new("from@example.org".to_string()).unwrap()),
+        vec![EmailAddress::new("to@example.org".to_string()).unwrap()],
+    ).unwrap();
+    let _email = EmailBuilder::new()
+        .envelope(e)
+        .subject("subject")
+        .text("message")
+        .build()
+        .unwrap();
+}