Use POST instead of PUT for merging Design Reviews

This bug wasn't caught because we didn't have a test before. I've added
one to test this case.

allspice/apiobject.py

@@ -2042,7 +2042,7 @@ class DesignReview(ApiObject):
     base: str
     body: str
     changed_files: int
-    closed_at: Any
+    closed_at: Optional[str]
     comments: int
     created_at: str
     deletions: int
@@ -2055,11 +2055,11 @@ class DesignReview(ApiObject):
     is_locked: bool
     labels: List[Any]
     merge_base: str
-    merge_commit_sha: Any
+    merge_commit_sha: Optional[str]
     mergeable: bool
     merged: bool
-    merged_at: Any
-    merged_by: Any
+    merged_at: Optional[str]
+    merged_by: Optional["User"]
     milestone: Any
     number: int
     patch_url: str
@@ -2165,7 +2165,7 @@ def merge(self, merge_type: MergeType):
         :param merge_type: The type of merge to perform. See the MergeType enum.
         """
 
-        self.allspice_client.requests_put(
+        self.allspice_client.requests_post(
             self.MERGE_DESIGN_REVIEW.format(
                 owner=self.repository.owner.username,
                 repo=self.repository.name,

tests/test_api.py

@@ -674,6 +674,11 @@ def test_create_design_review(instance):
     org = Organization.request(instance, test_org)
     repo = Repository.request(instance, org.username, test_repo)
     branch = Branch.request(instance, org.username, test_repo, "master")
+    repo.create_file(
+        "new_file.txt",
+        base64.b64encode(b"new file contents").decode("utf-8"),
+        {"branch": "test_branch"},
+    )
     due_date = datetime.datetime.now() + datetime.timedelta(days=7)
     review = repo.create_design_review(
         "TestDesignReview",
@@ -762,6 +767,33 @@ def test_get_design_review_attachments(instance):
     assert attachments[0].name == "requirements.txt"
 
 
+def test_merge_design_review(instance):
+    org = Organization.request(instance, test_org)
+    repo = Repository.request(instance, org.username, test_repo)
+    dr = repo.get_design_reviews()[0]
+
+    assert dr.state == "open"
+
+    # The API may fail with this message. It isn't in the scope of this test to
+    # make sure the API can merge, so we'll keep retrying until it works or we
+    # retry too many times.
+    attempts = 0
+    while attempts < 5:
+        try:
+            dr.merge(DesignReview.MergeType.MERGE)
+            break
+        except Exception as e:
+            if "Please try again later" in str(e):
+                attempts += 1
+                time.sleep(1)
+                continue
+            else:
+                raise e
+
+    dr = repo.get_design_reviews()[0]
+    assert dr.state == "closed"
+
+
 def test_repo_create_release(instance):
     org = Organization.request(instance, test_org)
     repo = Repository.request(instance, org.username, test_repo)