[11.5] Add deleteProtectedBranch method to projects api (#680)

* add deleteProtectedBranch method to projects api.

Handles the functionality to delete/unprotect a protected branch which
is documented here:
https://docs.gitlab.com/ee/api/protected_branches.html#unprotect-repository-branches

* Fixed param name

Co-authored-by: Graham Campbell <GrahamCampbell@users.noreply.github.com>

Author: matt-h

src/Api/Projects.php

@@ -1202,6 +1202,17 @@ public function addProtectedBranch($project_id, array $parameters = [])
         return $this->post($this->getProjectPath($project_id, 'protected_branches'), $parameters);
     }
 
+    /**
+     * @param int|string $project_id
+     * @param string     $branch_name
+     *
+     * @return mixed
+     */
+    public function deleteProtectedBranch($project_id, string $branch_name)
+    {
+        return $this->delete($this->getProjectPath($project_id, 'protected_branches/'.self::encodePath($branch_name)));
+    }
+
     /**
      * @param int|string $project_id
      *

tests/Api/ProjectsTest.php

@@ -2217,6 +2217,23 @@ public function shouldAddProtectedBranch(): void
         $this->assertEquals($expectedArray, $api->addProtectedBranch(1, ['name' => 'master', 'push_access_level' => 0, 'merge_access_level' => 30]));
     }
 
+    /**
+     * @test
+     */
+    public function shouldRemoveProtectedBranch(): void
+    {
+        $expectedBool = true;
+        $api = $this->getApiMock();
+        $api->expects($this->once())
+            ->method('delete')
+            ->with(
+                'projects/1/protected_branches/test-branch'
+            )
+            ->will($this->returnValue($expectedBool));
+
+        $this->assertEquals($expectedBool, $api->deleteProtectedBranch(1, 'test-branch'));
+    }
+
     /**
      * @test
      */