Created `BinIndex.Add` override for adding strings and checking for null.
Created `BinIndex.Add` override for adding strings and checking for null.


Former-commit-id: ef0f889e0b709b647e135dbfba65db52e2aea07f
basho/riak-dotnet-client,basho/riak-dotnet-client,rob-somerville/riak-dotnet-client,rob-somerville/riak-dotnet-client