title: java stream throws NPE when toMap stores a null value
body: <p><code>HashMap</code> allows to store <code>NULL</code> value, but <code>Stream.toMap(r-&gt; r.getName(), r-&gt;r.get(obj))</code> would throw NPE when <code>r.get(obj)</code> returns null? Do I miss something or <code>Stream</code> has a special reason to be more careful than <code>Map</code>? I am trying to use reflection and java8 to achieve <code>(new ObjectMapper()).convertValue(obj, Obj.class);</code></p>