mongodump fails when a collection name includes '/'

When you create a collection with a '/' in the collection name, mongodump will fail when exporting files

To reproduce create a database 'test' with a collection 'a/b'

With pymongo, this can be done by doing `getattr(db, 'a/b').save(

)

run mongodump; it will fail with this error:
> test.a/b to dump/test/a/b.bson
> assertion: userassert:couldn't open file stream invalid: errno:2 No such file or directory

the check for collection names should be stricter on access, or mongodump should handle them appropriately.