Copy headers across redirects

[germsvel]

Resolves #126

What changed?

In order to emulate browser behavior a little bit better, we want to copy headers across redirects.

Whenever we dispatch to the endpoint, the Phoenix.ConnTest.recycle/2 automatically gets called. But by default it only recycles "accept", "accept-language", and "authorization".

This commit calls it manually copying all request headers.

For more, see docs on
https://hexdocs.pm/phoenix/Phoenix.ConnTest.html#recycle/2

Recycling receives a connection and returns a new connection,
containing cookies and relevant information from the given one.

This emulates behaviour performed by browsers where cookies returned
in the response are available in following requests.

By default, only the headers "accept", "accept-language", and
"authorization" are recycled. However, a custom set of headers can be specified by passing a list of strings representing its names as the second argument of the function.

Note recycle/1 is automatically invoked when dispatching to the
endpoint, unless the connection has already been recycled.

Need to check the following:

• Redirect on visit
• Redirect when following links
• Redirect when following form submissions

[germsvel]

I wanted to make sure it wasn't unsafe to copy all the headers.

For posterity's sake, I think we're okay following these rules (snatched them from a friend):

• If POST 301 or POST 302, do not copy any headers
• If non-GET 303, do not copy any headers
• If redirecting to a different domain than the initial request, copy all headers EXCEPT Authorization
• Otherwise, copy all headers

Since we don't really handle 301s or 302s, I don't think we need to worry about that. I don't think people are redirecting to different domains (since we're presumably testing internal apps), so I don't think the issue of Authorization is a big deal.

So, I think we're okay copying all the headers for now.

I also gave Capybara (a Ruby library that has rack-test which is similar to Phoenix.ConnTest) a look to see how they'd handled this in the past. I found this issue (teamcapybara/capybara#464) which was resolved in this commit (teamcapybara/capybara@33b1a27) which I think basically agrees that it's fine to copy headers.

[joeytrapp]

Thank you for looking into this!