Change map_stream to use Hash#fetch

This moves the logic for whether to use a default key or not into the
Hash class, where it belongs. Tell, don't ask and all that.
Merge Stream#map_stream and Stream#map

map_stream does more or less the same thing as map, but with a little
bit of syntactic sugar.
Merge pull request #12 from henrikhodne/map-stream-fetch

Change map_stream to use Hash#fetch