Take spiv review comments into account.

* bzrlib/transport/http/response.py:
(RangeFile._seek_to_next_range): Factored out since this is now
used by both seek and read.
(RangeFile.read): Trigger next range recognition when needed.
(RangeFile.seek): Don't seek over the range boundary if not
required to.

* bzrlib/transport/http/__init__.py:
(HttpTransportBase._coalesce_readv.get_and_yield): Add a
prophylactic assertionError.

* bzrlib/tests/test_transport_implementations.py:
Fix typos.

* bzrlib/tests/test_http_response.py:
(TestRangeFileMixin.test_read_zero,
TestRangeFileMixin.test_seek_at_range_end,
TestRangeFileMixin.test_read_at_range_end,
TestRangeFileSizeUnknown.test_read_at_range_end,
TestRangeFilMultipleRanges.test_seek_at_range_end,
TestRangeFilMultipleRanges.test_read_at_range_end): More tests
covering read(0).