Append newline after each bulk request

[0x0badc0de]

Added a test at spec/chewy/type/import_spec.rb:73 which fails as following:

  1) Chewy::Type::Import.import :bulk_size 
     Failure/Error: result = client.bulk options.merge(header).merge(body: body)

     Elasticsearch::Transport::Transport::Errors::BadRequest:
       [400] {"error":{"root_cause":[{"type":"action_request_validation_exception","reason":"Validation Failed: 1: no requests added;"}],"type":"action_request_validation_exception","reason":"Validation Failed: 1: no requests added;"},"status":400}

Looks like to be known issue - need a newline at the end of bulk entry, see
https://discuss.elastic.co/t/elastic-search-bulk-post-testing/24065/4
http://stackoverflow.com/a/22997612
This PR adds missing newline so test passes.

[pyromaniac]

Awesome, thanks!