* Drop dependency + use built-in `base64url`
[`create-hmac`](https://github.com/browserify/createHmac) is meant for compat between node and browser compat and was last updated in 2018.
Computing the HMAC signature on the client (browser) does not make any sense in 99% of the use-cases, as the secrets would be needed on the client side.
This means that we can drop browser support and just use the native node module, which is also exported by the `create-hmac` when running on node.
`Buffer.toString()` also accepts "base64url" as an encoding, so we can drop the `urlSafeBase64` in favor of that.
* Use encoder from hmac instance
* Always return Last-Modified and support If-Modified-Since.
* IMGPROXY_USE_LAST_MODIFIED config setting.
IMGPROXY_USE_LAST_MODIFIED (default false) when enabled will return the
Last-Modified time of the upstream image and also allow the support of
the If-Modified-Since request header (returning a 304 if the image
hasn't been modified).
If-Modified-Since allows If-None-Match to take precedence.
* Fixes based on DarthSim's feedback.
1. Don't worry about nil maps.
2. Fix a test now that we use the config.LastModifiedEnabled (and move
it's location it he test file to a more sane place).
3. Update GCS transport code based on the refactoring of DarthSim.
In this iteration, we pull the Updated time from the GCS object attributes
and format them as a string. We then parse it in the notmodified module.
Seems a bit silly to do it this way. If we agree on the approach here,
then AWS and Azure can follow.
* Support azure, fs, s3, and swift.
* Grab the headers for If-Modified-Since and Last-Modified before parsing them.
* Add tests for last-modified for fs.
* Support Last-Modified being passed when streaming an upstream file.
* Tests for Last-Modified for GCS and Azure
* Support s3 and swift tests. Sadly fakes3 doesn't support Last-Modified
* Test against forked gofakes3
