If `pedantic_wpt` is enabled, preserve header case

See [`basic/request-headers-case.any.js`](https://github.com/web-platform-tests/wpt/blob/master/fetch/api/basic/request-headers-case.any.js). This one might not be worth doing even with the `pedantic_wpt` flag on, because it could break a lot of assumptions. If this is our conclusion, we can WONTFIX this issue.

We currently convert all header names to lowercase in order to canonicalize them. In practice HTTP header names aren't case sensitive. If a header name is repeated multiple times, the values are combined with a comma.

The fetch spec however requires us to preserve the case of the first occurrence of the header name.