staticcheck: warn when "Content-Length" or "Connection" is added to http.Request.Header

[vcabbage]

// For client requests, certain headers such as Content-Length
// and Connection are automatically written when needed and
// values in Header may be ignored. See the documentation
// for the Request.Write method.

https://golang.org/pkg/net/http/#Request.Header

Instead of setting Content-Length or Connection directly in the headers, Request.ContentLength and Request.Close should be used instead.

Match statements like:

request.Header.Set("Content-Length", ...)
request.Header.Add("Content-Length", ...)
request.Header.Set("Connection", "close")
request.Header.Add("Connection", "close")

Since headers are canonicalized, a case insensitive match of the header name would be ideal.

Matching entries when the header map is manually constructed would also be nice, but I imagine would be more involved.