CSHARP-1439: Code review changes.

Author: rstam

src/MongoDB.Driver.Core.Tests/Core/Operations/CreateCollectionOperationTests.cs

@@ -242,7 +242,7 @@ public void CreateCommand_should_return_expected_result_when_ValidationAction_is
             var expectedResult = new BsonDocument
             {
                 { "create", _collectionNamespace.CollectionName },
-                { "validationAction", value.ToString().ToLower() }
+                { "validationAction", value.ToString().ToLowerInvariant() }
             };
 
             var result = subject.CreateCommand();
@@ -262,7 +262,7 @@ public void CreateCommand_should_return_expected_result_when_ValidationLevel_is_
             var expectedResult = new BsonDocument
             {
                 { "create", _collectionNamespace.CollectionName },
-                { "validationLevel", value.ToString().ToLower() }
+                { "validationLevel", value.ToString().ToLowerInvariant() }
             };
 
             var result = subject.CreateCommand();

src/MongoDB.Driver.Core/Core/Operations/CreateCollectionOperation.cs

@@ -200,8 +200,8 @@ internal BsonDocument CreateCommand()
                 { "flags", () => _usePowerOf2Sizes.Value ? 1 : 0, _usePowerOf2Sizes.HasValue},
                 { "storageEngine", () => _storageEngine, _storageEngine != null },
                 { "validator", _validator, _validator != null },
-                { "validationAction", () => _validationAction.Value.ToString().ToLower(), _validationAction.HasValue },
-                { "validationLevel", () => _validationLevel.Value.ToString().ToLower(), _validationLevel.HasValue }
+                { "validationAction", () => _validationAction.Value.ToString().ToLowerInvariant(), _validationAction.HasValue },
+                { "validationLevel", () => _validationLevel.Value.ToString().ToLowerInvariant(), _validationLevel.HasValue }
             };
         }