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
