fix: handle multiple `x-forwarded-proto` headers (#58824)
### What?
This PR changes how protocol is determined.
A change was recently made (in [PR
57815](https://github.com/vercel/next.js/commit/1caa58087a428666bb14d40dbc159b9a768b075d#diff-c49c4767e6ed8627e6e1b8f96b141ee13246153f5e9142e1da03450c8e81e96fR1744))
that did not take into account cases where there are multiple
`x-forwarded-proto` headers. In such cases, the protocol becomes e.g
"https, https".
### Why?
An error will occur in parseUrl on line 1616, since its not a valid url
(e.g. `https, https://localhost:3000`).
### How?
Reverted part of the changes in [PR
57815](https://github.com/vercel/next.js/pull/57815).
Fixes #58764 and fixes #59031
Closes NEXT-2437
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Co-authored-by: Balázs Orbán <info@balazsorban.com>
