This enables `go get` to parse go-import meta tags from index.html
files stored in IPFS. One tiny step toward whyrusleeping/gx-go#2.

For an import like `ipfs.io/ipfs/QmFoo/mypkg`, the gateway would
previously redirect to `/ipfs/QmFoo/mypkg/` (note the trailing slash),
which the `go get` tool can't deal with.

Thankfully, `go get` sets a URL query parameter (`?go-get=1`) which
we can use to switch off the redirect in this case.

License: MIT
Signed-off-by: Lars Gierth <larsg@systemli.org>