Fix GitHub JSON and models

If the JSON would contain null, there is no way for the model generator
to identify the type, so it would incorrectly use String here.

Author: ePirat

src/api/models/github.rs

@@ -427,8 +427,8 @@ pub struct PullRequestPullRequest {
     pub author_association: Option<String>,
     pub draft: Option<bool>,
     pub merged: Option<bool>,
-    pub mergeable: Option<String>,
-    pub rebaseable: Option<String>,
+    pub mergeable: Option<bool>,
+    pub rebaseable: Option<bool>,
     pub mergeable_state: Option<String>,
     pub merged_by: Option<PullRequestPullRequestMergedBy>,
     pub comments: Option<i64>,

src/api/models/json/github.json

@@ -712,8 +712,8 @@
       "author_association": "OWNER",
       "draft": false,
       "merged": false,
-      "mergeable": null,
-      "rebaseable": null,
+      "mergeable": true,
+      "rebaseable": true,
       "mergeable_state": "unknown",
       "merged_by": {
         "login": "brndnmtthws",