Fix error message when there's an invalid byte sequence

Author: jeffcharles

src/r_string.rs

@@ -1100,7 +1100,7 @@ impl RString {
 
     unsafe fn as_str_unconstrained<'a>(self) -> Result<&'a str, Error> {
         self.test_as_str_unconstrained().ok_or_else(|| {
-            let msg: Cow<'static, str> = if self.is_utf8_compatible_encoding() {
+            let msg: Cow<'static, str> = if !self.is_utf8_compatible_encoding() {
                 format!(
                     "expected utf-8, got {}",
                     RbEncoding::from(self.enc_get()).name()

tests/string.rs

@@ -10,4 +10,14 @@ fn it_converts_to_utf8_string() {
     let s = String::try_convert(val).unwrap();
 
     assert_eq!("café", s);
+
+    let val: Value = magnus::eval!(r#""\xFF\xFF""#).unwrap();
+    let err = String::try_convert(val).unwrap_err();
+
+    let expected_error = "invalid byte sequence in UTF-8";
+    assert!(
+        err.to_string().contains(expected_error),
+        "Expected \"{}\" to contain \"{expected_error}\" but it didn't",
+        err.to_string()
+    );
 }