Convert super invocations to python3 style

I, the author of the code so far, still qualify as "more familiar with
Python2 than Python3" and as such I was unaware of the argumentless
super() builtin until recently. Now that I know of it, I figure it's
probably much more Pythonic to use the new style when writing Python3
code.
Fix minor error in the docstring for BaseTestCase

The docstring was errantly reporting that BaseTestCase set the settings
module to a None when in fact it was setting it to an empty settings
object. (That is, an object with no attributes.)
Make http sender more lenient with status lines

Some servers don't include a message in their responses making the
status line something like "HTTP/1.1 200" rather than "HTTP/1.1 200 Ok".
This change makes the http sender tolerant of such responses by setting
the "message" field in the response object to an empty string in such
cases.
Make chunked body test slightly more rigorous

This change ensures that the ChunkedBody works well with multiple reads.
Add one more test to the empty body test

This change ensures that the length of an EmptyBody object is always
zero.