Fix the PUT-existing bug in writable gateway API

[Stebalien]

MOVED FROM: #3931

@jes wrote:

See on https://discuss.ipfs.io/t/writeable-http-gateways/210

Previously, a PUT to a filename that already exists would return the hash
of the newly-inserted file instead of the hash of the new tree. This is not
useful, and it makes much more sense for the API to be consistent.

Furthermore, anybody who knows about the bug is already DELETE-ing the filename
before PUT-ing the new content, so the fix doesn't trample over that behaviour
either.

With this fix, every conceivable usage of the API is either unaffected or
improved.

It could do with a unit test.

---

All I did was remove the special-case for the "file already exists" case, and it works correctly. It's easier to see this if you view git diff -w.

[Stebalien]

This was previously reviewed by both @whyrusleeping and @Kubuxu. I believe I've addressed their remaining comments (#3931 (review)).

[Stebalien]

This looks like something we'll need to fix:

Tests / linuxSharness / 40 - We can HTTP GET file just replaced – sharnessLinux.t0111-gateway-writeable

Stacktrace
URL="http://localhost:$port/ipfs/$HASH/test/test.txt" && echo "GET $URL" && curl -svo outfile3 "$URL" 2>curl_getExisting.out && test_cmp infile3 outfile3
Standard Output
expecting success:
URL="http://localhost:$port/ipfs/$HASH/test/test.txt" &&
echo "GET $URL" &&
curl -svo outfile3 "$URL" 2>curl_getExisting.out &&
test_cmp infile3 outfile3
GET http://localhost:37288/ipfs/QmfGKo7mm41im56UKjRJFKYqJiTMhsmYcNazSYHSzGmxnF/test/test.txt
> diff -u infile3 outfile3
--- infile3 2019-01-24 15:34:02.691086012 +0000
+++ outfile3 2019-01-24 15:34:05.886928567 +0000
@@ -1 +1 @@
-8372
+24246
not ok 40 - We can HTTP GET file just replaced
Standard Error

[Stebalien]

Fixed in libp2p/go-libp2p-connmgr#50.