don't require ProviderName attribute when using <connectionStrings> (app.config etc)

[304NotModified]

Fixes #1115

Before was needed:

<add name="test_connectionstring_with_providerName" connectionString="some connectionstring" 

providerName="System.Data.SqlClient" <-----------------

/>

even if "DBProvider" was set. E.g.

                databaseTarget .DBProvider = "System.Data.SqlClient"

[codecov-io]

Current coverage is 75.01%

Merging #1317 into master will not affect coverage as of 0c54895

@@            master   #1317   diff @@
======================================
  Files          267     267       
  Stmts        15467   15471     +4
  Branches      1626    1627     +1
  Methods          0       0       
======================================
+ Hit          11602   11606     +4
  Partial        383     383       
  Missed        3482    3482       

Review entire Coverage Diff as of 0c54895

---

Uncovered Suggestions

1. +0.09% via ...gingConfiguration.cs#282...295
2. +0.09% via ...gingConfiguration.cs#248...261
3. +0.09% via ...argets/FileTarget.cs#1713...1725
4. See 7 more...

Powered by Codecov. Updated on successful CI builds.

[304NotModified]

documented on https://github.com/NLog/NLog/wiki/Database-target