Rack 3 support

[lsylvester]

Add support for rack 3 by relaxing the version constraints and downcasing all of the headers.

Fixes #753

[lsylvester]

This was a reverting of the lower case headers which caused a performance issue - but this was resolved in rack 2.2.4 according to #746 (comment)

[skipkayhil]

I think this will have the same issues as the commit that was reverted. If Sprockets want to support both Rack 2 and 3 then it will need to be able to handle both Case-Sensitive and downcase only headers.

[lsylvester]

I think this will have the same issues as the commit that was reverted. If Sprockets want to support both Rack 2 and 3 then it will need to be able to handle both Case-Sensitive and downcase only headers.

Rack 2 has case insensitive headers so the downcase headers should be should still be valid. The issue that caused the regression was a bug in rack that was resolved by rack/rack#1919. We may wish to lock rack to >= 2.2.4 (which I have added)- but I think that may be overly restrictive as I think it was only a performance regression in test (and maybe dev) - but not production. Not sure if this (either changing the case of headers that may be accessed elsewhere, or the restriction of rack versions) could be considered a breaking change.

[skipkayhil]

Rack 2 has case insensitive headers so the downcase headers should be should still be valid. The issue that caused the regression was a bug in rack that was resolved by rack/rack#1919

Rack fixed one instance of the issue, but as long as Sprockets claims compatability with Rack 2 it needs to work with any other library that supports Rack 2. I believe the casing of the headers needs to be dependent on the version of Rack, since we can't assume other libraries will have already updated.

[lsylvester]

but as long as Sprockets claims compatability with Rack 2 it needs to work with any other library that supports Rack 2

I do not think that this is correct. Any library using Rack 2 could make any assumptions about the case of the header that it likes - so maybe sprockets is already incompatible with existing Rack 2 middleware. I think it is reasonable it argue that this could be considered a breaking change and that we need to bump the major version of sprockets - so that app maintainers would know to check for any incompatibilities - but it should still be able to support both Rack 2 and 3 using the lower case headers.

The decision to bump the major version is something that I will leave to the maintainers of the project and I will not presume what they would consider a breaking change.

[rafaelfranca]

Yeah, this patch is fine since sprockets will require the version of rack to the update to 2.2.4 which allow headers to be head with both cases.

[ioquatix]

Thanks everyone!

[ghiculescu]

There's a user reported issue on Rack 2.2.6 that seems related to this, could someone take a look? rails/rails#47096