net/textproto: Customize to call the function 'CanonicalMIMEHeaderKey'

[angYuHao]

What version of Go are you using (go version)?

$ go version
go version go1.17.3 windows/amd64

Does this issue reproduce with the latest release?

yes

The key of token in the request header of many servers is nonstandard. When I want to proxy the server restful api, the function 'CanonicalMIMEHeaderKey' cause me trouble
The token keyword carried by my request is 'AuthToken'
After the function CanonicalMIMEHeaderKey, it becomes 'Authtoken'
the server can't identify 'Authtoken'

[seankhliao]

use the header map directly.

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