Add _foreign_keys connection parameter

Fixes #377
Updates #255

sqlite3_test.go

@@ -107,6 +107,35 @@ func TestReadonly(t *testing.T) {
 	}
 }
 
+func TestForeignKeys(t *testing.T) {
+	cases := map[string]bool{
+		"?_foreign_keys=1": true,
+		"?_foreign_keys=0": false,
+	}
+	for option, want := range cases {
+		fname := TempFilename(t)
+		uri := "file:" + fname + option
+		db, err := sql.Open("sqlite3", uri)
+		if err != nil {
+			os.Remove(fname)
+			t.Errorf("sql.Open(\"sqlite3\", %q): %v", uri, err)
+			continue
+		}
+		var enabled bool
+		err = db.QueryRow("PRAGMA foreign_keys;").Scan(&enabled)
+		db.Close()
+		os.Remove(fname)
+		if err != nil {
+			t.Errorf("query foreign_keys for %s: %v", uri, err)
+			continue
+		}
+		if enabled != want {
+			t.Errorf("\"PRAGMA foreign_keys;\" for %q = %t; want %t", uri, enabled, want)
+			continue
+		}
+	}
+}
+
 func TestClose(t *testing.T) {
 	tempFilename := TempFilename(t)
 	defer os.Remove(tempFilename)