README: adjust boost::optional example

The "from_json" example for boost::optional is not complete and should also handle the 'none' case.

README: adjust boost::optional example
f2873e6d · Jared Grubb · GitHub · f642934b · f2873e6d
Commit f2873e6d authored Jan 29, 2017 by Jared Grubb Committed by GitHub Jan 29, 2017
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README.md README.md +3 -1

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--- a/README.md
+++ b/README.md
@@ -563,7 +563,9 @@ namespace nlohmann {
        }

        static void from_json(const json& j, boost::optional<T>& opt) {
-            if (!j.is_null()) {
+            if (j.is_null()) {
+                opt = boost::none;
+            } else {
                opt = j.get<T>(); // same as above, but with 
                                  // adl_serializer<T>::from_json
            }