Do not double encode encoded slashes

Do not double encode encoded slashes

In case that AllowEncodedSlashes is set to NoDecode do not double encode
encoded slashes in the URL sent by the reverse proxy to the backend.

* include/ap_mmn.h: Document the addition of ap_proxy_canonenc_ex to the API.

* modules/proxy/mod_proxy.h: Declare ap_proxy_canonenc_ex and define flag
      values.

* modules/proxy/proxy_util.c: Implement ap_proxy_canonenc_ex by modifying
      ap_proxy_canonenc accordingly and reimplement ap_proxy_canonenc to
      use ap_proxy_canonenc_ex with the appropriate flag.

* modules/http2/mod_proxy_http2.c, modules/proxy/mod_proxy_*.c: Set the
      correct flag based on the AllowEncodedSlashes configuration and use
      ap_proxy_canonenc_ex instead of ap_proxy_canonenc.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1908341 13f79535-47bb-0310-9956-ffa450edef68