Use WriteConcern.SAFE as default for MongoDB connection

[vshchukin]

Hi,

We are using nosql-unit to insert/read test data with remote MongoDB.
But data defined in @UsindDataSet sometimes is not available in test for reading.
The problem is that default WriteConcern in Java driver does not guarantee this - so I think it's better to always use WriteConcern.SAFE .

[buildhive]

Alex Soto » nosql-unit #43 SUCCESS
This pull request looks good
(what's this?)

[lordofthejars]

Thank you very much first of all for using NoSQLUnit and second one for helping me in improving it because you are right with the write concern problem. Tomorrow I will merge your changes.
Again thank you so much for your interest in project, and if you have any request don't hesitate to ask me. Also let me tell you that in two or three weeks I will release a version for testing MongoDB replication (shards, master/slave and replication sets.)