[Sys.Configuration] Remove a misleading MonoTODO in ConnectionStrings

The ConnectionStrings property of System.Configuration.ConfigurationManager
works perfectly, so this MonoTODO is not needed.

A unit test is also added to demonstrate that it's fully working and to
prevent it failing in the future.
[Sys.Configuration] Remove unneeded NET_2_0 ifdefs
Merge pull request #644 from knocte/connstrings

 [System.Configuration] Remove a misleading MonoTODO in ConnectionStrings