`pflag.GetStringSlice` treats commas as delimiters, which causes issues
when passing headers whose values contain commas (`X-Robots-Tag:
noindex, nofollow`). These are incorrectly split into multiple headers
and errors out:
- `X-Robots-Tag: noindex`
- ` nofollow`
Switch to `pflag.GetStringArray`, which does not split on commas[1].
Note that this changes behavior for cases where multiple headers were
provided in a single argument with commas (`--header-down "X-Foo:
Bar,X-Bar: Foo"`). Such cases will now be treated as a single header
value. If this breaking change is unacceptable, we will need a smarter
fallback mechanism.
[1] https://github.com/spf13/pflag/pull/90