Ignore ContainsMetadata field when automapping Attachment (#4789)

This commit adds [Ignore] attribute to the ContainsMetadata
field to ignore the field when automapping an Attachment.

[IgnoreDataMember] ignores the value when serializing an
instance of Attachment, but NoopPropertyVisitor does not
take into account [IgnoreDataMember] when mapping properties.

Author: russcam

src/Nest/Mapping/Types/Specialized/Attachment/Attachment.cs

@@ -29,6 +29,7 @@ public class Attachment
 		/// attachment.
 		/// </summary>
 		[IgnoreDataMember]
+		[Ignore]
 		public bool ContainsMetadata =>
 			!Author.IsNullOrEmpty() ||
 			ContentLength.HasValue ||

src/Nest/Mapping/Visitor/NoopPropertyVisitor.cs

@@ -6,6 +6,7 @@
 
 namespace Nest
 {
+	// TODO: Make all methods virtual
 	public class NoopPropertyVisitor : IPropertyVisitor
 	{
 		public virtual bool SkipProperty(PropertyInfo propertyInfo, ElasticsearchPropertyAttributeBase attribute) => false;