fix bug for value checking of MinPoolSize

Hi Francisco,

In the following sample,Npgsql will fail currently.

var conn = new NpgsqlConnection(ConnectionString + ";MinPoolSize=30;MaxPoolSize=30");//*1
conn.Open();

*1) here will throw the following  error:
System.ArgumentOutOfRangeException : MINPOOLSIZE
Parameter name: numeric value MINPOOLSIZE in ConnectionString exceeds maximum value 20

I send the patch to fix the bug,please check it. 
btw,the error message above also mistook Parameter name and message. So I also modified it.