Replace a Header.Get call with getHeaderCanonical (#682)

This is the only one I see left, and this is one is in the hot path for
every request.

duplex_http_call.go

@@ -290,7 +290,7 @@ func (d *duplexHTTPCall) makeRequest() {
  defer close(d.responseReady)
 
  // Promote the header Host to the request object.
- if host := d.request.Header.Get(headerHost); len(host) > 0 {
+ if host := getHeaderCanonical(d.request.Header, headerHost); len(host) > 0 {
  d.request.Host = host
  }
  if d.onRequestSend != nil {