maxLookup index out of range on update #54

Hi,

Upon adding a new column to a table that was already large I started experiencing index out of range at

https://github.com/mgutz/dat/blob/v1/update.go#L178

It seems the condition that triggers hand off between preallocated parameter conversions and ad hoc conversions may be off by one. If I change i < maxLookup to i < maxLookup - 1 everything works fine but not sure if that's right, as the last element in equalsPlaceholderTab will not be used. Also, is it right that equalsPlaceholderTab starts with $0?

I wrote a little snippet to test for various values of maxLookup (https://github.com/mgutz/dat/blob/v1/init.go#L19):

type TestStruct struct {
	ID     int64  `db:"id"`
	Field1 string `db:"field1"`
	Field2 string `db:"field2"`
}

func TestBreak() {
	DB.SQL(`CREATE TABLE testtable (id serial, field1 text, field2 text);`).Exec()
	DB.SQL(`INSERT INTO testtable (field1, field2, field3) VALUES ('a', 'b');`).Exec()

	record := TestStruct{}
	_, err := DB.
		Update("testtable").
		SetWhitelist(record, "*").
		Where("id = $1", 1).
		Exec()
	if err != nil {
		panic(err)
	}

	DB.SQL(`DROP TABLE testtable;`).Exec()
}

This breaks for maxLookup <= 3 but works for larger values.

Cheers,

Luis