[Twisted-web] Tests in #3384 fix

[Aaron DeVore]

I am working on completing the patch for ticket #3384
(HTTPClientFactory should follow relative redirects). The actual code
is complete but the tests are not. I need some input as to how many
tests I should write. If I write a test for every possible case I
would have about 10 new tests and 40 new lines of code.

The tests would be a mix of the following:
- Absolute path (starts at the root directory on the server) versus
relative path (starts at the current directory)
- With(out) a trailing slash in factory.url
- With(out) non-path component (a mix of param, query, and fragment as
returned by urlparse.urlparse)

Should I be this thorough?

-Aaron