debugger: validate sec-websocket-accept response header

[Trott]

First commit by @copperwall:

debugger: validate sec-websocket-accept response header

This addresses a TODO to validate that the sec-websocket-accept header
in the websocket handshake response is valid. To do this we need to
append the Websocket GUID to the original key sent in sec-websocket-key,
sha1 hash it, and then compare the base64 encoding with the value sent
in the sec-websocket-accept response header.

If they don't match, an error is thrown.

Second commit:

test: add test for websocket secret verification in debugger

Refs: nodejs/node-inspect#93

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/38981/

[cjihrig]

LGTM. Is "WebSocket" the correct capitalization of the word? I could be wrong, but thought I'd ask since you're using "Websocket" throughout this PR.

[Trott]

Is "WebSocket" the correct capitalization of the word? I could be wrong, but thought I'd ask since you're using "Websocket" throughout this PR.

Fixed it in the commit messages and in the code added here.