Throw instead of ignore, if headers are invalid

[emmenko]

After spending on hour trying to debug the problem, I found out that I was passing a number as header value, thus the header was ignored.

I think it would be much more useful throwing an error if something is not valid, instead of silently ignore it.

Thanks

[coveralls]

Coverage remained the same at 100.0% when pulling 62c0de4 on emmenko:validate-headers into 60f819f on bitinn:master.

[bitinn]

I would like to see how browsers and github polyfill handles it before making changes.

I would like to avoid throwing errors if possible, especially on value like null and undefined

[bitinn]

I have pushed a version 1.2.1 to normalize Number type instead (only for new Headers at the moment, you need to take care using .set or .append)

[emmenko]

@bitinn perfect, that would do it as well! Thanks 👍