types: make project readme_url optional

Projects with no readme have no such url.

src/test/types.rs

@@ -366,7 +366,7 @@ fn test_read_project() {
     assert_eq!(project.web_url,
                "https://gitlab.kitware.com/utils/rust-gitlab");
     assert_eq!(project.readme_url,
-               "https://gitlab.kitware.com/utils/rust-gitlab/blob/master/README.md");
+               Some("https://gitlab.kitware.com/utils/rust-gitlab/blob/master/README.md".to_string()));
     assert!(project.owner.is_none());
     assert_eq!(project.name, "rust-gitlab");
     assert_eq!(project.name_with_namespace, "Utils / rust-gitlab");

src/types.rs

@@ -530,7 +530,7 @@ pub struct Project {
     /// The URL for the project's homepage.
     pub web_url: String,
     /// The URL for the project's readme.
-    pub readme_url: String,
+    pub readme_url: Option<String>,
     /// The owner of the project (`None` for a group-owned project).
     pub owner: Option<UserBasic>,
     /// The display name of the project.