Addresses US 152

see below for the default constructor should simply be spelled constexpr. The current declaration looks like a member function, not a constructor, and the constexpr keyword implicitly does not apply unless the instantiation could make it so, under the guarantees al ready present in the Effects clause.

Proposed change:

Replace see below with constexpr in the declaration of the default constructor for istream_iterator in the class definition, and function specification.