[HTML5] Fix JavaScript string parsing with new interface.

Strings are UTF-8 encoded and should be parsed as such, while it was
being parsed as a C string before.

(cherry picked from commit d4e302a3a499b4a3445b4e1610757fc310d1641d)

platform/javascript/javascript_singleton.cpp

@@ -183,7 +183,7 @@ Variant JavaScriptObjectImpl::_js2variant(int p_type, godot_js_wrapper_ex *p_val
 		case Variant::REAL:
 			return p_val->r;
 		case Variant::STRING: {
-			String out((const char *)p_val->p);
+			String out = String::utf8((const char *)p_val->p);
 			free(p_val->p);
 			return out;
 		}