merge r3452, r3462, r3471, r3472:

merge r3452, r3462, r3471, r3472:

fix miscellaneous name bugs:

*) fix $upstream_http_ variable prefix length
*) use a right "Location" header name, however, it did not harm,
   since ngx_http_variable_sent_location() never use key name field
*) fix proxy_redirect name in error message
*) "proxy_redirect default" may not be used if a proxy_pass uses variables