Skip serializing default GitHub host (#88)

The configuration enables users to set a custom endpoint for GitHub.
This can be useful for users of GitHub Enterprise, but is mostly used
to mock GitHub in tests.

The setting is now excluded from the rendered configuration if it
points to github.com, which cleans up the configuration.

src/cli/configuration/mod.rs

@@ -249,7 +249,6 @@ mod tests {
             r#"
             library:
               github:
-                instance: https://api.github.com/
                 owner: jdno
                 repository: flowcrafter
             workflows:

src/github/configuration.rs

@@ -10,6 +10,7 @@ use crate::github::repository::Repository;
 #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
 pub struct GitHubConfiguration {
     #[cfg_attr(feature = "serde", serde(default = "default_instance"))]
+    #[cfg_attr(feature = "serde", serde(skip_serializing_if = "is_default_instance"))]
     #[builder(default = default_instance())]
     instance: Url,
 
@@ -44,6 +45,10 @@ fn default_instance() -> Url {
     Url::parse("https://api.github.com").expect("failed to parse hard-coded GitHub URL 🤯")
 }
 
+fn is_default_instance(instance: &Url) -> bool {
+    instance == &default_instance()
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -101,6 +106,44 @@ mod tests {
         assert_send::<GitHubConfiguration>();
     }
 
+    #[cfg(feature = "serde")]
+    #[test]
+    fn trait_serialize_with_default_instance() {
+        let configuration = GitHubConfiguration::builder()
+            .owner("jdno")
+            .repository("flowcrafter")
+            .build();
+
+        let yaml = indoc!(
+            r#"
+            owner: jdno
+            repository: flowcrafter
+            "#
+        );
+
+        assert_eq!(yaml, serde_yaml::to_string(&configuration).unwrap());
+    }
+
+    #[cfg(feature = "serde")]
+    #[test]
+    fn trait_serialize_with_custom_instance() {
+        let configuration = GitHubConfiguration::builder()
+            .instance(Url::parse("https://github.example.com").unwrap())
+            .owner("jdno")
+            .repository("flowcrafter")
+            .build();
+
+        let yaml = indoc!(
+            r#"
+            instance: https://github.example.com/
+            owner: jdno
+            repository: flowcrafter
+            "#
+        );
+
+        assert_eq!(yaml, serde_yaml::to_string(&configuration).unwrap());
+    }
+
     #[test]
     fn trait_sync() {
         fn assert_sync<T: Sync>() {}