Add sum_other_doc_count to bucket agg response

Closes #1041
Fix NullReferenceException in AggregationsHelper
when bucket is not available in aggregations.
Bump version to 1.2.3
Merge tag '1.2.3'

1.2.3

Conflicts:
	src/Nest/Properties/AssemblyInfo.cs
	src/Tests/Nest.Tests.Integration/Properties/AssemblyInfo.cs