fix impacted tests

Author: shiftkey

LibGit2Sharp.Tests/ConfigurationFixture.cs

@@ -256,7 +256,7 @@ public void CanSetBooleanValue()
         [Fact]
         public void SettingLocalConfigurationOutsideAReposThrows()
         {
-            using (var config = new Configuration())
+            using (var config = new Configuration(null, null, null))
             {
                 Assert.Throws<LibGit2SharpException>(() => config.Set("unittests.intsetting", 3));
             }

LibGit2Sharp/Configuration.cs

@@ -26,7 +26,7 @@ public class Configuration : IDisposable,
         /// <summary>
         /// Default constructor, should only be used for mocking purposes
         /// </summary>
-        public Configuration() : this(null, null, null)
+        protected Configuration()
         { }
 
         internal Configuration(Repository repository, string globalConfigurationFileLocation,