CORS middleware fixed and refactored

[LuisEGR]

While I was working with servest and Angular I noticed that the current implementation of CORS is not fully working(I am still getting CORS error), this is because the headers are only added to the preflight request (OPTIONS) and there are three headers that need to also be sent in all the other allowed methods as well (access-control-allow-origin, access-control-expose-headers and access-control-allow-credentials), adding this solves the problem, but to avoid code duplication I did a refactor of the entire file so it is more readable.

• Fixes CORS missing headers (in other methods distinct to OPTIONS)
• Fixes Typo in header accessl-control-expose-headers -> access-control-expose-headers
• Added more description in the documentation blocks

Sources:
CORS spec: https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS
Express CORS: https://github.com/expressjs/cors/blob/c49ca10e92ac07f98a3b06783d3e6ba0ea5b70c7/lib/index.js

[keroxp]

Oh, it's really significant improvement! Thanks @LuisEGR !

[LuisEGR]

It is a pleasure to help!