Add option to specify cache headers

[jacobwegner]

By default, GCS caches any publicly accessible objects for 3600 seconds.

Especially if a developer is trying to update an existing object (see #4), it may not be desirable to have objects with that long of a cache value.

Either add an option to GoogleCloudStorage to specify cache headers or an additional backend that supports custom cache headers.

GCS_PUBLIC_READ_CACHE_DEFAULT = "public, max-age=3600"

...

class CustomCacheMediaStorage(GoogleCloudStorage):
    """
    Custom subclass of GoogleCloudStorage that allows customization of GCS's
    cache-control header
    """
    cache_control = GCS_PUBLIC_READ_CACHE_DEFAULT

    def _save(self, name, content):
        mimetype, _ = mimetypes.guess_type(os.path.basename(name))
        if mimetype is None:
            mimetype = "application/octet-stream"
        media = MediaIoBaseUpload(content, mimetype)
        req = self.client.objects().insert(
            bucket=self.bucket,
            name=self._prefixed_name(name),
            body={
                "cacheControl": self.cache_control
            },
            media_body=media
        )
        req.execute()
        return name

...

class NoCacheMediaStorage(CustomCacheMediaStorage):
    # never cache
    cache_control = "private, max-age=0"

[jacobwegner]

@brosner any thoughts about how we want to implement this? Perhaps add a GAPC_STORAGE key for cacheControl?

[jacobwegner]

@brosner: would like to revisit your thoughts on this and #4 for the next release

[brosner]

I am in favor of adding a setting to control this behavior. cache_control key in GAPC_STORAGE sounds right to me.