test: cleanup vars to const and '==' to '==='

Modified test-http-upgrade-client.js

Line 29: Changed var port to const port
Line 30: Changed var headers to const headers
Line 57: Changed assert.equal to assert.strictEqual
Line 61: Changed var expectedHeaders to const
Line 69: Changed '==' to '===' comparison

PR-URL: https://github.com/nodejs/node/pull/8705


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Teddy Katz <teddy.katz@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>