DatabaseTarget should also dispose connection on close

[dodexahedron]

Fixed CloseConnection() leaks IDbConnection objects as reported in #609

[304NotModified]

The unit tests won't expect the dispose . Can you fix that?

Thanks!

[dodexahedron]

I'll have a look.

[304NotModified]

FYI: It's not a difficult issue, some tests at NLog.UnitTests.Targets.DatabaseTargetTests are failing, because they read the log.

For example:

NLog.UnitTests.Targets.DatabaseTargetTests.KeepConnectionOpenBatchedTest2 [FAIL]
   Assert.Equal() Failure
   Position: First difference is at position 188
   Expected: Open('Database=MyLogger').
ExecuteNonQuery: INSERT INTO FooBar VALUES('msg1')
ExecuteNonQuery: INSERT INTO FooBar VALUES('msg2')
ExecuteNonQuery: INSERT INTO FooBar VALUES('msg4')
Close()
Open('Database=MyLogger2').
ExecuteNonQuery: INSERT INTO FooBar VALUES('msg3')

   Actual:   Open('Database=MyLogger').
ExecuteNonQuery: INSERT INTO FooBar VALUES('msg1')
ExecuteNonQuery: INSERT INTO FooBar VALUES('msg2')
ExecuteNonQuery: INSERT INTO FooBar VALUES('msg4')
Close()
Dispose() <!-------------------------------------------------------------------------- NEW 
Open('Database=MyLogger2').
ExecuteNonQuery: INSERT INTO FooBar VALUES('msg3')

[dodexahedron]

All unit tests in that class pass, now.

[304NotModified]

Thanks!