`form_urlencoded::serialize(...)` handles owned as well as borrowed pairs

[Byron]

This is the result of a long path full of (my own) trial and error on how to allow serialize() not to be hardcoded to owned tuples, and increase usability.

I have added some tests which prove it now works both ways, without altering the existing API in any way (i.e. this is not a breaking change).

Even though discussed in #81, I didn't removed serialize_owned(), as it's possibly breaking and I would want to leave that to the superior judgement of the maintainers.

[SimonSapin]

the superior judgement of the maintainers.

These are strong words, when the maintainers often feel like this :)

Regardless, I’m a bit reluctant to add complexity to this signature, when it seems the only reason borrowed tuples are around in the first place is that [T; N] doesn’t implement IntoIterator. See also #81 (comment)

[Byron]

Well, at least I suspect the maintainers have less 'no idea of what they are doing' than me :).

If until Rust beta, [T; N] should implement IntoIterator (and I am sceptic this will happen), my change indeed shouldn't be necessary.
One could wait with the merge until then.

Personally I think the complexity in the signature is just what Rust is, and should be something we all get more and more accustomed to. Also I prefer adding complexity to the implementation to keep it light and simple on the caller's end. The needs of the many outweigh the needs of the few :) !

#81 comment will be addressed there.