folly::dynamic::operator= for std::initializer_list

Summary:
If we're going to have a constructor for it, we should have operator=.

Otherwise, gcc-4.9 goes via the copy constructor effectively "ignoring" the attempted nesting for e.g..

```
d = { other_dynamic };  // Should be ARRAY containing dynamic
```

NOTE: this only fixes gcc-4.9, there's still issues in clang.

Reviewed By: yfeldblum

Differential Revision: D2745180

fb-gh-sync-id: 667787c788fc7c131d8a34c608c355f5b875be50

folly/dynamic-inl.h

@@ -282,6 +282,11 @@ inline dynamic::dynamic(fbstring&& s)
   new (&u_.string) fbstring(std::move(s));
 }
 
+inline dynamic& dynamic::operator=(std::initializer_list<dynamic> il) {
+  (*this) = dynamic(il);
+  return *this;
+}
+
 inline dynamic::dynamic(std::initializer_list<dynamic> il)
   : type_(ARRAY)
 {

folly/dynamic.h

@@ -167,6 +167,7 @@ public:
    *   dynamic v = { 1, 2, 3, "foo" };
    */
   /* implicit */ dynamic(std::initializer_list<dynamic> il);
+  dynamic& operator=(std::initializer_list<dynamic> il);
 
   /*
    * Conversion constructors from most of the other types.