Replace key 'Source' by 'source'

osm.org/browse/changeset/7850813
- 50 ways changed

osm.org/browse/changeset/7850799
- 82 ways changed

osm.org/browse/changeset/7850781
- 59 nodes changed
- 54 ways changed

osm.org/browse/changeset/7850751
- 89 nodes changed
- 94 ways changed