[#25] Handle 304 redirects

Problem: after recent work on Xrefcheck redirect behavior, it remained
to discuss how to handle 304 redirects.

Solution: with the current default configuration, 304 redirects are
considered as valid, which seems appropriate taking into account that
304 responses usually mean that you previously received a successful
response for that same request and there is no need to retransmit the
resource again. We add a test case for this default config and update
the FAQ section.

README.md

@@ -143,7 +143,7 @@ There are several ways to fix this:
 1. How does `xrefcheck` handle redirects?
     * The rules from the default configuration are as follows:
       * Permanent redirects (i.e. 301 and 308) are reported as errors.
-      * Temporary redirects (i.e. 302, 303 and 307) are assumed to be valid.
+      * There are no rules for other redirects, except for a special GitLab case, so they are assumed to be valid.
     * Redirect rules can be specified with the `externalRefRedirects` parameter within `networking`, which accepts an array of
       rules with keys `from`, `to`, `on` and `outcome`. The rule applied is the first one that matches with
       the `from`, `to` and `on` fields, if any, where

tests/Test/Xrefcheck/RedirectDefaultSpec.hs

@@ -23,7 +23,7 @@ import Xrefcheck.Verify
 
 test_redirectRequests :: TestTree
 test_redirectRequests = testGroup "Redirect response defaults"
-  [ testGroup "Temporary" $ temporaryRedirectTests <$> [302, 303, 307]
+  [ testGroup "Temporary and 304" $ allowedRedirectTests <$> [302, 303, 304, 307]
   , testGroup "Permanent" $ permanentRedirectTests <$> [301, 308]
   ]
   where
@@ -33,8 +33,8 @@ test_redirectRequests = testGroup "Redirect response defaults"
     location :: Maybe Text
     location = Just "http://127.0.0.1:5000/other"
 
-    temporaryRedirectTests :: Int -> TestTree
-    temporaryRedirectTests statusCode =
+    allowedRedirectTests :: Int -> TestTree
+    allowedRedirectTests statusCode =
       redirectTests
         (show statusCode <> " passes by default")
         (mkStatus statusCode "Temporary redirect")