Escape keyword strings in ast.Ref#String

Previously refs like p["not"] would be represented as p.not which does
not parse because 'not' is an invalid variable name. This change ensures
that ref string operands are escape if they are keywords.

ast/term.go

@@ -910,7 +910,7 @@ func (ref Ref) String() string {
 		switch p := p.Value.(type) {
 		case String:
 			str := string(p)
-			if varRegexp.MatchString(str) && len(buf) > 0 {
+			if varRegexp.MatchString(str) && len(buf) > 0 && !IsKeyword(str) {
 				buf = append(buf, "."+str)
 			} else {
 				buf = append(buf, "["+p.String()+"]")

ast/term_test.go

@@ -311,6 +311,7 @@ func TestTermString(t *testing.T) {
 	assertToString(t, RefTerm(VarTerm("foo"), StringTerm("bar")).Value, "foo.bar")
 	assertToString(t, RefTerm(VarTerm("foo"), StringTerm("bar"), VarTerm("i"), IntNumberTerm(0), StringTerm("baz")).Value, "foo.bar[i][0].baz")
 	assertToString(t, RefTerm(VarTerm("foo"), BooleanTerm(false), NullTerm(), StringTerm("bar")).Value, "foo[false][null].bar")
+	assertToString(t, RefTerm(VarTerm("p"), StringTerm("not")).Value, `p["not"]`)
 	assertToString(t, ArrayTerm().Value, "[]")
 	assertToString(t, ObjectTerm().Value, "{}")
 	assertToString(t, SetTerm().Value, "set()")