Use more appropriate istreambuf_iterator in the streaming.cc example

Used in the example std::istream_iterator skips whitespace and thus
a parser can produce unexpected reult, e.g.:

 Input: [{"x":"test space","y":20}]
 Output: "testspace",20

std::istreambuf_iterator does not skip whitespace, so is more
appropriate:

 Output: "test space",20

More info:
http://en.cppreference.com/w/cpp/iterator/istream_iterator (see "Notes"
section).

examples/streaming.cc

@@ -64,7 +64,7 @@ int main(void) {
   root_context ctx;
   std::string err;
 
-  picojson::_parse(ctx, std::istream_iterator<char>(std::cin), std::istream_iterator<char>(), &err);
+  picojson::_parse(ctx, std::istreambuf_iterator<char>(std::cin), std::istreambuf_iterator<char>(), &err);
 
   if (!err.empty()) {
     std::cerr << err << std::endl;