Use POST instead of PUT for cancelMergeRequest(..) (#948)

Fixes #931

src/main/java/org/gitlab4j/api/MergeRequestApi.java

@@ -804,7 +804,7 @@ public class MergeRequestApi extends AbstractApi {
      *
      * <p>NOTE: GitLab API V4 uses IID (internal ID), V3 uses ID to identify the merge request.</p>
      *
-     * <pre><code>GitLab Endpoint: PUT /projects/:id/merge_requests/:merge_request_iid/cancel_merge_when_pipeline_succeeds</code></pre>
+     * <pre><code>GitLab Endpoint: POST /projects/:id/merge_requests/:merge_request_iid/cancel_merge_when_pipeline_succeeds</code></pre>
      *
      * @param projectIdOrPath the project in the form of an Long(ID), String(path), or Project instance
      * @param mergeRequestIid the internal ID of the merge request
@@ -817,7 +817,7 @@ public class MergeRequestApi extends AbstractApi {
             throw new RuntimeException("mergeRequestIid cannot be null");
         }
 
-        Response response = put(Response.Status.OK, null, "projects", getProjectIdOrPath(projectIdOrPath), "merge_requests", mergeRequestIid, "cancel_merge_when_pipeline_succeeds");
+        Response response = post(Response.Status.OK, (Form)null, "projects", getProjectIdOrPath(projectIdOrPath), "merge_requests", mergeRequestIid, "cancel_merge_when_pipeline_succeeds");
         return (response.readEntity(MergeRequest.class));
     }