fix(validation): allow image height up to spec maximum of 400

Image height was validated against 0..=144, the same bound as width, but the
RSS 2.0 specification allows heights up to 400 (width max is 144). Spec-valid
images with a height between 145 and 400 were wrongly reported as invalid.
Validate height against 0..=400 and fix the error message.

tests/validation.rs

@@ -0,0 +1,42 @@
+#![cfg(feature = "validation")]
+
+use rss::validation::Validate;
+use rss::ImageBuilder;
+
+fn image_with_height(height: &str) -> rss::Image {
+    ImageBuilder::default()
+        .url("https://example.com/image.png".to_string())
+        .link("https://example.com/".to_string())
+        .title("title".to_string())
+        .height(height.to_string())
+        .build()
+}
+
+fn image_with_width(width: &str) -> rss::Image {
+    ImageBuilder::default()
+        .url("https://example.com/image.png".to_string())
+        .link("https://example.com/".to_string())
+        .title("title".to_string())
+        .width(width.to_string())
+        .build()
+}
+
+#[test]
+fn image_height_allows_up_to_400() {
+    // The RSS 2.0 specification sets the maximum image height to 400.
+    assert!(image_with_height("31").validate().is_ok());
+    assert!(image_with_height("200").validate().is_ok());
+    assert!(image_with_height("400").validate().is_ok());
+}
+
+#[test]
+fn image_height_above_400_is_invalid() {
+    assert!(image_with_height("401").validate().is_err());
+}
+
+#[test]
+fn image_width_max_is_still_144() {
+    // Width keeps its specification maximum of 144.
+    assert!(image_with_width("144").validate().is_ok());
+    assert!(image_with_width("145").validate().is_err());
+}