NpgsqlConnectionStringBuilder: Fix KeyNotFoundException for Keywords.ApplicationName

Creating an instance of `NpgsqlConnectionStringBuilder` would lead to a `KeyNotFoundException` in `NpgsqlConnectionStringBuilder.SetValue(string, Keyword, object)` due to a missing value description entry.

Example case to reproduce the error condition:

```
var connectionStr = new NpgsqlConnectionStringBuilder {
            Host = _host,
            UserName = _username,
            Password = _password,
            ApplicationName = "foobar"  // << offending code
};
```