Add head() method to return full HTTP headers

[fredmnl]

• Add new head() method that returns complete headers object
• Keeps existing getHEAD() for backward compatibility (returns only x-scal-usermd)

[bert-e]

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[bert-e]

Incorrect fix version

The Fix Version/s in issue SPRXCLT-21 contains:

• None

Considering where you are trying to merge, I ignored possible hotfix versions and I expected to find:

• 8.1.2

Please check the Fix Version/s of SPRXCLT-21, or the target
branch of this pull request.

[borisfaure]

Suggested change

	getHEAD(key, reqUids, callback) {
	getUsermd(key, reqUids, callback) {

Change the API?

[fredmnl]

Yeah, I was wondering whether I should simply change the API and break the clients (they'll just have to update their code), or mark it as deprecated first.

@borisfaure @jonathan-gramain What do you think?

[borisfaure]

Even though the lib is public, the only known users are Scality. Updating the caller at the same time the lib is bumped looks easy and solid enough to take the opportunity for a more readable API.