database/sql: accept nil pointers to Valuers implemented on value receivers

database/sql: accept nil pointers to Valuers implemented on value receivers

The driver.Valuer interface lets types map their Go representation to
a suitable database/sql/driver.Value.

If a user defines the Value method with a value receiver, such as:

    type MyStr string

    func (s MyStr) Value() (driver.Value, error) {
        return strings.ToUpper(string(s)), nil
    }

Then they can't use (*MyStr)(nil) as an argument to an SQL call via
database/sql, because *MyStr also implements driver.Value, but via a
compiler-generated wrapper which checks whether the pointer is nil and
panics if so.

We now accept (*MyStr)(nil) and map it to "nil" (an SQL "NULL")
if the Valuer method is implemented on MyStr instead of *MyStr.

If a user implements the driver.Value interface with a pointer
receiver, they retain full control of what nil means:

    type MyStr string

    func (s *MyStr) Value() (driver.Value, error) {
        if s == nil {
            return "missing MyStr", nil
        }
        return strings.ToUpper(string(*s)), nil
    }

Adds tests for both cases.

Fixes #8415

Change-Id: I897d609d80d46e2354d2669a8a3e090688eee3ad
Reviewed-on: https://go-review.googlesource.com/31259
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Theophanes <kardianos@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>