Add content type headers to raw KV responses

[picatz]

CVE-2020-25864

A vulnerability was identified in Consul and Consul Enterprise ("Consul") such that a specially crafted KV entry could be used to perform a XSS attack when viewed in the raw mode.

This PR adds content-type headers to raw KV responses to prevent that attack.

[hashicorp-ci]

🤔 This PR has changes in the website/ directory but does not have a type/docs-cherrypick label. If the changes are for the next version, this can be ignored. If they are updates to current docs, attach the label to auto cherrypick to the stable-website branch after merging.

[rboyer]

LGTM

[hashicorp-ci]

🍒 If backport labels were added before merging, cherry-picking will start automatically.

To retroactively trigger a backport after merging, add backport labels and re-run https://circleci.com/gh/hashicorp/consul/350888.

[hashicorp-ci]

🍒✅ Cherry pick of commit 9f7190a onto release/1.9.x succeeded!

[hashicorp-ci]

🍒❌ Cherry pick of commit 9f7190a onto release/1.8.x failed! Build Log

[hashicorp-ci]

🍒❌ Cherry pick of commit 9f7190a onto release/1.7.x failed! Build Log