Fix error message when parsing unknown JSON enum value

`#string_value` can be the value of the next token, which results in a
confusing error message.

spec/std/json/serialization_spec.cr

@@ -302,6 +302,9 @@ describe "JSON serialization" do
         expect_raises(JSON::ParseException, %(Unknown enum JSONSpecEnum value: "three")) do
           JSONSpecEnum.from_json(%("three"))
         end
+        expect_raises(JSON::ParseException, %(Unknown enum JSONSpecEnum value: "three")) do
+          NamedTuple(foo: JSONSpecEnum).from_json(%({"foo": "three", "other": 1}))
+        end
         expect_raises(JSON::ParseException, %(Expected String but was Int)) do
           JSONSpecEnum.from_json(%(1))
         end

src/json/from_json.cr

@@ -318,11 +318,13 @@ def Enum.new(pull : JSON::PullParser)
   {% if @type.annotation(Flags) %}
     value = {{ @type }}::None
     pull.read_array do
-      value |= parse?(pull.read_string) || pull.raise "Unknown enum #{self} value: #{pull.string_value.inspect}"
+      string = pull.read_string
+      value |= parse?(string) || pull.raise "Unknown enum #{self} value: #{string.inspect}"
     end
     value
   {% else %}
-    parse?(pull.read_string) || pull.raise "Unknown enum #{self} value: #{pull.string_value.inspect}"
+    string = pull.read_string
+    parse?(string) || pull.raise "Unknown enum #{self} value: #{string.inspect}"
   {% end %}
 end