Add support for setting custom headers for files uploaded to S3

[hannseman]

Today there's only support for setting the Cache-Control header. I've removed the S3_CACHE_CONTROL setting and added S3_HEADERS which supports setting whatever header you want:

S3_HEADERS = {
    'Expires': 'Thu, 15 Apr 2010 20:00:00 GMT',
    'Cache-Control': 'max-age=86400',
}

[hannseman]

Bump @e-dard :)

I've also got a commit fixing an issue where setting the header Content-Encoding: gzip was set on all files uploaded and not only files with gzippable mime-types:
hannseman/flask-s3@ab3333bad1c5ca47b2820f9802420169fe98e1ba

But we'll take in another PR if you want to merge this one.

[e-dard]

OK, finally got some time to sit down and look at this 😄 I've been completely off-base with Flask for while, as my new gig is mainly a Ruby shop...

Unfortunately I merged the previous PR with too much haste, and I really wish I hadn't 💣. The solution you propose, here, is clearly a much better alternative. However, now I've gone and sullied my once relatively tidy extension, I'm going to have to live with it.

Your PR will break Flask S3 as it stands, because existing deployments may (will) be using S3_CACHE_CONTROL and S3_USE_CACHE_CONTROL, so it's going to need some modifications. Here are my suggestions:

• put back the cache-based defaults to maintain backwards compatibility;
• update the documentation for those defaults stating that they are deprecated, and S3_HEADERS should be used instead;
• On instantiation of the FlaskS3 object use the values of S3_USE_CACHE_CONTROL and S3_CACHE_CONTROL to decide if you need to set those keys in S3_HEADERS.

I will add some line comments to clarify the above.

Don't worry if these changes are too much to ask, I'll get around to them eventually. I promise a quick review if you decide to do them, though 😆

[e-dard]

For both of these settings I would simply add

Deprecated. Please use S3_HEADERS instead.

[hannseman]

No problem! I understand that you need backwards compatibility, somehow I managed to not think about that. I'll give it a shot and come back to you.

[e-dard]

The code block looks gnarly in the table for some reason. Thinking about it, it may be better just to keep the documentation here to:

Sets custom headers to be sent with each file to S3.
Default: {}

And then after Uploading your Static Assets add another very short section: Setting Custom HTTP Headers, outlining how to use S3_HEADERS (along with relavant links — on that note can you use the foo_ and .. _foo http://bar.com notation?)

[e-dard]

Thanks @hannseman!

[hannseman]

Thanks for the merge! @e-dard