Fix: token filter type assertion

[AnneYang720]

What this PR does:
Fix type assertion in token filter.
remoteTkn must be assert to []string[0], because of http2 header limitation.

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[Mulavar]

I'm sorry that issue #1998 has been fixed, the purpose of this pr is the same as the other pr #1999 .
I saw your pr, and maybe you forgot that the token filter should be compatible with the dubbo protocol too, so we need to check both type assertion with string and []string.
@AnneYang720