exp/sql: rename NullableString to NullString and allow its use as a parameter

Prep for Issue 2699

R=rsc
CC=golang-dev
https://golang.org/cl/5536045
diff --git a/src/pkg/exp/sql/fakedb_test.go b/src/pkg/exp/sql/fakedb_test.go
index 70aa68c..0376583 100644
--- a/src/pkg/exp/sql/fakedb_test.go
+++ b/src/pkg/exp/sql/fakedb_test.go
@@ -589,7 +589,9 @@
 	case "int32":
 		return driver.Int32
 	case "string":
-		return driver.String
+		return driver.NotNull{driver.String}
+	case "nullstring":
+		return driver.Null{driver.String}
 	case "datetime":
 		return driver.DefaultParameterConverter
 	}