Allow iterator copy for lambda in transformed and filtered

Lambda has no default constructor nor a copy or move assignment.

range\test\adaptors.cpp is checking that produced ranges are copiable,
but it is not doing so the for lambda, or lambda like objects.

default_constructible_unary_fn uses an optional for default
construction.
I extended it to use its emplace facility to copy assign non copy
assignable types.
(see boost::optional 'Type Requierment':  'If T is not MoveAssignable,
it is still possible to reset the value of optional<T> using function
emplace():')

include/boost/range/detail/default_constructible_unary_fn.hpp

@@ -32,6 +32,23 @@ public:
         : m_impl(source)
     {
     }
+    default_constructible_unary_fn_wrapper(const default_constructible_unary_fn_wrapper& source)
+        : m_impl(source.m_impl)
+    {
+    }
+    default_constructible_unary_fn_wrapper& operator=(const default_constructible_unary_fn_wrapper& source)
+    {
+        if (source.m_impl)
+        {
+            // Lambda are not copy/move assignable.
+            m_impl.emplace(*source.m_impl);
+        }
+        else
+        {
+            m_impl.reset();
+        }
+        return *this;
+    }
     template<typename Arg>
     R operator()(const Arg& arg) const
     {