improved documentation and test coverage

doc/examples/flatten.cpp

@@ -31,4 +31,7 @@ int main()
 
     // call flatten()
     std::cout << std::setw(4) << j.flatten() << '\n';
+
+    // flatten for a primitive value
+    std::cout << j["pi"].flatten() << '\n';
 }

doc/examples/flatten.link

@@ -1 +1 @@
-<a target="_blank" href="http://melpon.org/wandbox/permlink/kODXfzcksgstdBRD"><b>online</b></a>
+<a target="_blank" href="http://melpon.org/wandbox/permlink/skGi8b32VhI8HOgV"><b>online</b></a>

doc/examples/flatten.output

@@ -14,3 +14,4 @@
     "/object/~1": "slash",
     "/pi": 3.141
 }
+{"":3.141}