Correcting ability to set guid Key, as it was overwritten when saving.

Adding unit tests to verify get/set Key.
Adding unit tests to verify GetById with guid Key.
Adding overload to GetById with guid Key to supplement the int Id.

src/Umbraco.Core/Models/Content.cs

@@ -327,7 +327,9 @@ namespace Umbraco.Core.Models
         internal override void AddingEntity()
         {
             base.AddingEntity();
-            Key = Guid.NewGuid();
+
+            if(Key == Guid.Empty)
+                Key = Guid.NewGuid();
         }
 
         /// <summary>

src/Umbraco.Core/Services/ContentService.cs

@@ -77,6 +77,20 @@ namespace Umbraco.Core.Services
             return repository.Get(id);
         }
 
+        /// <summary>
+        /// Gets an <see cref="IContent"/> object by its 'UniqueId'
+        /// </summary>
+        /// <param name="key">Guid key of the Content to retrieve</param>
+        /// <returns><see cref="IContent"/></returns>
+        public IContent GetById(Guid key)
+        {
+            var repository = RepositoryResolver.ResolveByType<IContentRepository, IContent, int>(_unitOfWork);
+            var query = Query<IContent>.Builder.Where(x => x.Key == key);
+            var contents = repository.GetByQuery(query);
+            return contents.SingleOrDefault();
+        }
+
+
         /// <summary>
         /// Gets a collection of <see cref="IContent"/> objects by the Id of the <see cref="IContentType"/>
         /// </summary>
@@ -740,7 +754,7 @@ namespace Umbraco.Core.Services
                 //If a user id was passed in we use that
                 content.CreatorId = userId;
             }
-            else if(_userService != null)
+            else if (UserServiceOrContext())
             {
                 var profile = _httpContext == null
                                   ? _userService.GetCurrentBackOfficeUser()
@@ -766,7 +780,7 @@ namespace Umbraco.Core.Services
                 //If a user id was passed in we use that
                 content.WriterId = userId;
             }
-            else if (_userService != null)
+            else if (UserServiceOrContext())
             {
                 var profile = _httpContext == null
                                   ? _userService.GetCurrentBackOfficeUser()
@@ -780,6 +794,11 @@ namespace Umbraco.Core.Services
             }
         }
 
+        private bool UserServiceOrContext()
+        {
+            return _userService != null && (HttpContext.Current != null || _httpContext != null);
+        }
+
         //TODO Add method to remove versions from Content
         //TODO Add method to remove versions from all Content - parameters to select date-interval, ea. remove versions older then.
     }

src/Umbraco.Core/Services/IContentService.cs

@@ -31,6 +31,13 @@ namespace Umbraco.Core.Services
         /// <returns><see cref="IContent"/></returns>
         IContent GetById(int id);
 
+        /// <summary>
+        /// Gets an <see cref="IContent"/> object by its 'UniqueId'
+        /// </summary>
+        /// <param name="key">Guid key of the Content to retrieve</param>
+        /// <returns><see cref="IContent"/></returns>
+        IContent GetById(Guid key);
+
         /// <summary>
         /// Gets a collection of <see cref="IContent"/> objects by the Id of the <see cref="IContentType"/>
         /// </summary>