remove my incorrect claim that we returned 400 in the wrong case

     2.4.x patch: http://people.apache.org/~ylavic/httpd-2.4.x-ap_map_http_request_error-v2.patch

     +1: ylavic, minfrin

     ylavic: Depends on r1657881 above.

             Comment from trawick below is about r1643537+r1643543+r1674056 which

             was first proposed separately (and now included in this proposal).

     trawick: Eric and I were working through this and I (at least) ran into this

              particular roadblock: output_failed sometimes means that we couldn't

              write to the client and sometimes means that we couldn't read from

              the client; later, output_fails triggers setting the status to 400.

              HTTP status shouldn't be 400 for a problem writing to the client.

              (Maybe I missed some guarantee that 400 is only for an error reading

              enough request body??)  I think it would be good to separate output_failed

              from input_failed so that the code is more clear and we can more

              easily tell that the status code is appropriate.  (And unfortunately

              I don't volunteer to make the simple changes and test them :( )

     ylavic: I think the confusion comes from the (original) name output_failed itself,

             since it really means client_failed (i.e. "dialog with [client] %pI failed"),

             as opposed to backend_failed (i.e. "dialog with backend %pI failed").

             When an error occurs on any side (connection), the first flag used regarding

             4xx/5xx vs DONE is data_sent (i.e. to the client), otherwise {4xx,5xx} are

             returned according to {output,backend}_failed. So the clarification is

             possibly as simple as renaming output_failed => client_failed (r1674056 added

             above), WDYT?

     trawick: It still looks to me that an error with ap_pass_brigade (towards

              client) can turn into a 400 error, which is what I was concerned

              about originally.

  *) http: Don't remove the Content-Length of zero from a HEAD response if

     it comes from an origin server, module or script. Allow the previous