fix return from std::map bindings to __delitem__ (#1229)

Fix return from `std::map` bindings to `__delitem__`: we should be returning `void`, not an iterator.

Also adds a test for map item deletion.

include/pybind11/stl_bind.h

@@ -587,7 +587,7 @@ class_<Map, holder_type> bind_map(handle scope, const std::string &name, Args&&.
                auto it = m.find(k);
                if (it == m.end())
                    throw key_error();
-               return m.erase(it);
+               m.erase(it);
            }
     );