Fix numeric to float64 conversion

A previous optimization in 50933c0 had a logic bug when converting
pgtype.Numeric to a float64:

```
	if src.Exp == 1 {
		return float64(src.Int.Int64()), nil
	}
```

An exponent of one means multiply by 10.

#210

numeric.go

@@ -448,8 +448,11 @@ func (src *Numeric) toFloat64() (float64, error) {
 		return math.Inf(-1), nil
 	}
 
-	if src.Exp == 1 {
+	switch {
+	case src.Exp == 0:
 		return float64(src.Int.Int64()), nil
+	case src.Exp == 1:
+		return float64(src.Int.Int64() * 10), nil
 	}
 
 	buf := make([]byte, 0, 32)

numeric_test.go

@@ -385,6 +385,7 @@ func TestNumericEncodeDecodeBinary(t *testing.T) {
 		123,
 		0.000012345,
 		1.00002345,
+		50.000000,
 		math.NaN(),
 		float32(math.NaN()),
 		math.Inf(1),
@@ -442,3 +443,22 @@ func TestNumericSmallNegativeValues(t *testing.T) {
 		t.Fatalf("expected %s, got %s", "-0.000123", s)
 	}
 }
+
+// https://github.com/jackc/pgtype/issues/210
+func TestNumericFloat64(t *testing.T) {
+	n := pgtype.Numeric{
+		Int:    big.NewInt(5),
+		Exp:    1,
+		Status: pgtype.Present,
+	}
+
+	var f float64
+	err := n.AssignTo(&f)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if f != 50.0 {
+		t.Fatalf("expected %s, got %f", "50", f)
+	}
+}