Pass basic swift headers to Storlets

[Takashi Kajinami]

storlet_handler now only passes metadata to Storlet application, but In some cases we want to use basic swift headers.
For example, in image processing application, we want to know Content-Type to decide the way to decode the image data in swift.

[Takashi Kajinami (kajinamit)]

I'm also wondering if we should be able to overwrite basic headers some cases.

For example, when we overwrite object content with Storlet, the value in ETag header becomes different from the one calculated based on the response content. (This causes error in "swift download" because in swift command it checkes ETag value)
If storlets can overwrite basic headers, then we can replace ETag value by new one, and old ETag value as another header, something like Original-ETag.

[Hamdi Roumani (roumani)]

I think there are definitely use cases for this scenario ! Modifying the ETAG in particular (which would make this more transparent to other applications that validate the ETAG ... currently any storlet that changes the content-length will indeed cause any etag validation to fail)

btw user headers are covered through:
https://bugs.launchpad.net/storlets/+bug/1497578