CSHARP-1892: Fix a scenario that was not using the full dotted field name for an embedded field.

Author: rstam

src/MongoDB.Driver/Linq/Expressions/FieldExpression.cs

@@ -50,6 +50,15 @@ public FieldExpression(Expression document, string fieldName, IBsonSerializer se
             _original = original;
         }
 
+        public FieldExpression(FieldExpression other, IBsonSerializer serializer)
+        {
+            Ensure.IsNotNull(other, nameof(other));
+            _document = other.Document;
+            _fieldName = other.FieldName;
+            _original = other.Original;
+            _serializer = Ensure.IsNotNull(serializer, nameof(serializer));
+        }
+
         public Expression Document
         {
             get { return _document; }

src/MongoDB.Driver/Linq/Processors/SerializationBinder.cs

@@ -218,7 +218,7 @@ protected override Expression VisitUnary(UnaryExpression node)
                                 return new DocumentExpression(serializer);
                             case ExtensionExpressionType.Field:
                                 return new FieldExpression(
-                                    ((FieldExpression)serializationExpression).FieldName,
+                                    (FieldExpression)serializationExpression,
                                     serializer);
                             case ExtensionExpressionType.FieldAsDocument:
                                 return new FieldAsDocumentExpression(