Fixing ParameterMarkerFormat value of the schema collection

Hi.

I made a tiny patch to fix common schema collection related issue.

A value of "ParameterMarkerFormat" column of "DataSourceInfo" schema collection which is returned from NpgsqlConnection.GetSchema() is not valid. It must be usable as a format parameter for String.Format method to make a parameter placeholder on a SQL. So, its value should be ":{0}" rather than "{0}".

This pull request contains a metadata XML file fix and an unit test method for this issue.