Make some Release fields optional (#495)

Author: BenFradet

github4s/src/main/scala/github4s/domain/Repository.scala

@@ -69,14 +69,14 @@ final case class Release(
     draft: Boolean,
     prerelease: Boolean,
     created_at: String,
-    published_at: String,
+    published_at: Option[String],
     author: Option[User],
     url: String,
     html_url: String,
     assets_url: String,
     upload_url: String,
-    tarball_url: String,
-    zipball_url: String
+    tarball_url: Option[String],
+    zipball_url: Option[String]
 )
 
 final case class Content(

github4s/src/test/scala/github4s/utils/TestData.scala

@@ -265,14 +265,14 @@ trait TestData {
     draft = false,
     prerelease = false,
     created_at = "2011-04-10T20:09:31Z",
-    published_at = "2011-04-10T20:09:31Z",
+    published_at = Some("2011-04-10T20:09:31Z"),
     author = Some(user),
     url = githubApiUrl,
     html_url = githubApiUrl,
     assets_url = githubApiUrl,
     upload_url = githubApiUrl,
-    tarball_url = githubApiUrl,
-    zipball_url = githubApiUrl
+    tarball_url = Some(githubApiUrl),
+    zipball_url = Some(githubApiUrl)
   )
 
   val content = Content(