https://maps.googleapis.com/maps/api/place/search/json?location=28.635704,77.224982&radius=50000&types=restaurant&language=en&name=&sensor=false&key=
I guess the problem is the categorization applied by the owners of the places. When I search for "restaurant delhi" on google-maps I get a lot of restaurants, but none of them is categorized as "restaurant", it's lodging, restaurants-indian,Restaurants - Non - Vegetarian or something else, but never restaurant .
Instead of requesting the types restaurant search for the name restaurant , you'll get a bunch of results.
When you take a look at these results, you'll see, the types of the most results is set to "establishment" , not "restaurant"