remove underscore attrs when indexing/creating

[douglasroeder]

When using the Bulk API for actions like 'index' or 'create' we are not removing the set of reserved keywords (@underscored) attributes from the document, this PR just apply the 'drop_underscored()' function. Is there a reason for that? I'm not an expert on ES, so forgive me if I'm missing something.

[OpakAlex]

I think we don't need remove this attributes.

[douglasroeder]

@OpakAlex I've forgot to mention, I did it because my application running on ES 2.4 was raising this exception when indexing documents with attributes like '_parent':

 MapperParsingException[Field [_parent] is a metadata field and cannot be added inside a document. Use the index API request parameters.]

Not sure if it's a version thing, but I reckon those attributes are not allowed on any circumstances, let me know your thoughts

[douglasroeder]

I did some further investigation adding an acceptance test using my other PR (support for extra mapping settings) and it's weird because it did work with the '_parent' attribute as part of the document when indexing it. I'll wait for that PR to be approved and add some acceptance tests for Tirexs.Bulk module.

[Zatvobor]

awesome. Thanks