Fixes the do not follow redirects bug

[MiguelLavigne]

The HttpEngine would not obey the set state because there was no
correlation between the HttpURLConnection.setInstanceFollowRedirects
method and the OkHttpClient.

Fixed the missing link by adding a new setFollowRedirects method to the
OkHttpClient class. Bridged the gap for the HttpURLConnection.setInstanceFollowRedirects
by forwarding that state into the OkHttpClient.

Now the HttpEngine will always obey the OkHttpClient redirect state when
attempting the followUp phase.

Added necessary test to both OkUrlFactoryTest and CallTest.

#943

[MiguelLavigne]

While it's easy to create a bug a figured I could also attempt to fix it. Here's my attempt, hopefully it can be useful!

[MiguelLavigne]

Should I be worried of having a broken build?
I noticed in the build history that the oraclejdk8 job often fails. Is that common?

[JakeWharton]

Java 8 often fails. Flaky tests suck!

[JakeWharton]

Can you copy the Javadoc from the above setter and modify it to describe this behavior?

[MiguelLavigne]

Of course, not a problem. It's done!

[JakeWharton]

Nice tests.

[MiguelLavigne]

I thought it might be flakiness.
I updated the code with the comment. Let me know if there's anything else.
Thx!

[JakeWharton]

LGTM. @swankjesse?

[swankjesse]

I think I might tweak this sentence later once this is landed. I thought in OkHttp following redirects was the default.

[davidschreiber]

This PR does not add support for follow-up POST redirects. Currently the redirect is prevented in case of a POST request:

case HTTP_PERM_REDIRECT:
case HTTP_TEMP_REDIRECT:
    // "If the 307 or 308 status code is received in response to a request other than GET
    // or HEAD, the user agent MUST NOT automatically redirect the request"
    if (!userRequest.method().equals("GET") && !userRequest.method().equals("HEAD")) {
        return null;
    }

So only GET and HEAD redirects are supported. Am I wrong, or should POST requests should also fall through?

[danielgomezrico]

"If the 307 or 308 status code is received in response to a request other than GET
or HEAD, the user agent MUST NOT automatically redirect the request"

Why?

[swankjesse]

It’s now-obsolete advice actually.

   The set of request methods that are safe to automatically redirect is
   no longer closed; user agents are able to make that determination
   based upon the request method semantics.  The redirect status codes
   301, 302, and 307 no longer have normative requirements on response
   payloads and user interaction.  (Section 6.4)

https://tools.ietf.org/html/rfc7231#appendix-B

[danielgomezrico]

@swankjesse then no redirect is done by OkHttp with any http code?

I tried with OkHttp 3.4.1 but:

• 301 / 302 change POST to GET
• 307 / 308 there's no redirect, just the body with the redirect Uri

[swankjesse]

Nope, we follow redirects. Just the spec was recently revised and we could track the changes.

[danielgomezrico]

@swankjesse will be cool some sample or entry on FAQ for this :)