added two test cases and minor change to the documentation for single…

… file collaboration (#425)

Box.V2.Test.Integration/BoxCollaborationsManagerTestIntegration.cs

@@ -59,6 +59,49 @@ public async Task CollaborationsOnFolderWorkflow_LiveSession_ValidResponse()
             Assert.IsTrue(success, "Collaboration deletion was unsucessful");
         }
 
+        // Test to add collaboration by Box User ID and Box Group ID. 
+        [TestMethod]
+        public async Task AddGroupCollaboration_File_Fields_ValidResponse()
+        {
+            const string fileId = "238288183114";
+            const string groupId = "176708848";
+            const string userId = "349294186";
+
+            // Add Group Collaboration
+            BoxCollaborationRequest addGroupRequest = new BoxCollaborationRequest()
+            {
+                Item = new BoxRequestEntity()
+                {
+                    Id = fileId,
+                    Type = BoxType.file
+                },
+                AccessibleBy = new BoxCollaborationUserRequest()
+                {
+                    Type = BoxType.group,
+                    Id = groupId
+                },
+                Role = "viewer"
+            };
+
+            //Add User Collaboration
+            BoxCollaborationRequest addUserRequest = new BoxCollaborationRequest()
+            {
+                Item = new BoxRequestEntity()
+                {
+                    Id = fileId,
+                    Type = BoxType.file
+                },
+                AccessibleBy = new BoxCollaborationUserRequest()
+                {
+                    Type = BoxType.user,
+                    Id = userId
+                },
+                Role = "editor"
+            };
+            var groupFileCollaboration = await _client.CollaborationsManager.AddCollaborationAsync(addGroupRequest, notify: false);
+            var userFileCollaboration = await _client.CollaborationsManager.AddCollaborationAsync(addUserRequest, notify: false);
+        }
+
         [TestMethod]
         public async Task CollaborationsOnFileWorkflow_LiveSession_ValidResponse()
         {

Box.V2/Managers/BoxCollaborationsManager.cs

@@ -19,7 +19,7 @@ public BoxCollaborationsManager(IBoxConfig config, IBoxService service, IBoxConv
             : base(config, service, converter, auth, asUser, suppressNotifications) { }
 
         /// <summary>
-        /// Used to add a collaboration for a single user or a single group to a folder. 
+        /// Used to add a collaboration for a single user or a single group to a folder or file. 
         /// Either an email address, a user ID, or a group id can be used to create the collaboration. 
         /// If the collaboration is being created with a group, access to this endpoint is granted based on the group's invitability_level.
         /// </summary>

Box.V2/Models/Request/BoxCollaborationRequest.cs

@@ -9,7 +9,7 @@ public class BoxCollaborationRequest : BoxRequestEntity
     {
         /// <summary>
         /// The item to add the collaboration on
-        /// The ID and Type are required. The Type MUST also be folder
+        /// The ID and Type are required. The Type can be folder or file.
         /// </summary>
         [JsonProperty(PropertyName = "item")]
         public BoxRequestEntity Item { get; set; }