Optimize toJson() Performance

Summary:
Removing string::reserve() which causes O(n^2) penalty.

Fixes #477
Closes https://github.com/facebook/folly/pull/478

Reviewed By: yfeldblum

Differential Revision: D3850509

Pulled By: Orvid

fbshipit-source-id: ecf44c35b2aedadc5385d23c325cacab1abfd02d

folly/json.cpp

@@ -693,7 +693,6 @@ void escapeString(
     return c < 10 ? c + '0' : c - 10 + 'a';
   };
 
-  out.reserve(out.size() + input.size() + 2);
   out.push_back('\"');
 
   auto* p = reinterpret_cast<const unsigned char*>(input.begin());