Expose doc_as_upsert in bulk operaion api

[CCheSumo]

doc_as_upsert is documented here https://www.elastic.co/guide/en/elasticsearch/reference/2.3/docs-update.html#_literal_doc_as_upsert_literal

Ideally we should make BulkOperation a trait, and have BulkCreateOperation, BulkUpdateOperation, BulkDeleteOperation etc. Waiting for the 6.x branch cut to introducing this breaking change, issue #152.

It is a small enough change, I am going to merge it now to block our development. Will be happy to incorporate any review feedbacks in a separate PR.

[codecov-io]

Codecov Report

Merging #157 into master will decrease coverage by 0.2%.
The diff coverage is 20%.

@@             Coverage Diff              @@
##             master     #157      +/-   ##
============================================
- Coverage     61.08%   60.87%   -0.21%     
+ Complexity       89       88       -1     
============================================
  Files            14       14              
  Lines           961      961              
  Branches        236      236              
============================================
- Hits            587      585       -2     
- Misses          217      218       +1     
- Partials        157      158       +1

Impacted Files	Coverage Δ	Complexity Δ
...c/elasticsearch/akkahelpers/BulkIndexerActor.scala	52.94% <0%> (ø)	13 <0> (ø)	⬇️
...ologic/elasticsearch/restlastic/dsl/IndexDsl.scala	71.42% <50%> (ø)	0 <0> (ø)	⬇️
...lasticsearch/akkahelpers/ScanAndScrollSource.scala	59.37% <0%> (-3.13%)	8% <0%> (-1%)

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