remove x- from our headers

[dbu]

rfc6648 says to not prefix headers with x- but if anything use a vendor prefix. lets change everything to fos- instead of x-

[ddeboer]

However, that RFC also states that new header creators ‘SHOULD NOT prefix their parameter names with "X-" or similar constructs’. I guess fos- would be a similar construct. The idea seems to be that we should assume that any of our headers could become general in principle. So perhaps get rid of the prefix altogether?

[dbu]

you are right. these are just the default names, so if somebody indeed does run into a naming conflict, they can simply change the defaults we provide. lets do that then.

[dbu]

see discussion in #285 - we will keep the prefix to avoid an unnecessary upgrading gotcha.