Test DbDataReader.GetX conversions

[bgrainger]

Test whether DbDataReader.GetX(n) performs an implicit conversion if GetFieldType(n) != typeof(X).

For example, calling GetString on a FLOAT column, or GetInt16 on a BIGINT column.

This will help inform decisions such as mysql-net/MySqlConnector#408.

[bgrainger]

Added numeric/string conversions in 3d8dbc6.