Add support for Content-Encoding for objects

[jmwski]

To support this change, which is documented at https://devhub.joyent.com/jira/browse/MANTA-3440, we want to add a --encoding option to the mmpu create command and allow a 'content-encoding' header to be stored in the upload record and then copied into the target object record on mmpu commit.

[davepacheco]

Sorry if this is a dumb question, but is there any reason we can't use the existing "-H/--header" option for this?

[jmwski]

In the codepath for mmpu create, the options (--size, --copies, --md5) take precedence over equivalent values that are added with -H. I think the reason for this was to separate headers that apply to the request itself with headers that describe the target object.

[jordanhendricks]

The flags --size, --copies, and --md5 are provided mostly as a convenience for the CLI user. The values passed in with the flag only take precedence if the same header is added using "-H": e.g., mmpu create --size 5 -H 'content-length:10' ~~/stor/foo would store "content-length": 5 on the target object. I would expect using "-H" to work as a way to provide the content-encoding header -- if it doesn't, that's a bug.

That said, I'm not opposed to having an additional flag to making specifying the encoding more convenient, as we do with content-{length,md5} and durability-level. But it shouldn't be strictly needed to store the header.