fix: return None for Gamma::mode when α < 1

src/distribution/gamma.rs

@@ -236,15 +236,15 @@ impl Mode<Option<f64>> for Gamma {
     /// # Formula
     ///
     /// ```text
-    /// max{(α - 1) / β, 0}
+    /// (α - 1) / β, where α > 1
     /// ```
     ///
     /// where `α` is the shape and `β` is the rate
     fn mode(&self) -> Option<f64> {
-        if self.shape > 1.0 {
+        if self.shape >= 1.0 {
             Some((self.shape - 1.0) / self.rate)
         } else {
-            Some(0.0)
+            None
         }
     }
 }