* Update index.d.ts
add proxyOption.proxyReq
* Update index.d.ts
proxyRes and proxyReq can be Function or Function[]
* Update index.d.ts
* Update browser-sync-tests.ts
add test for proxyRes & proxyReq
* Update browser-sync-tests.ts
These weak type errors were not caught in TS 2.4 RC. The final TS 2.4
will catch weak type errors with primitives, so this PR fixes those
now-caught errors.