BUGFIX: Remove length check on ga.send_data

Now that we're using generators we can't easily test whether there are
any documents without reading into the generator. It also doesn't give
very much value.

It may be useful in the future for the post method on DataSet to return
the number of documents that it sent.
Merge pull request #90 from alphagov/remove-length-check

BUGFIX: Remove length check on ga.send_data