Rewrite requests that access multiple indices

Hi,

Kibana (latest HEAD, I don't know if it did that in previous versions) did a query on the following URL:

  /logstash-2013.11.13,logstash-2013.11.12/_search

kibana3_auth didn't properly secure it:

  /logstash-2013.11.13,logstash-2013.11.12_ab1beaceb12e73819376633cfa3d2e7a16b29bf1

This commit fixes this:

  /logstash-2013.11.13_ab1beaceb12e73819376633cfa3d2e7a16b29bf1,logstash-2013.11.12_ab1beaceb12e73819376633cfa3d2e7a16b29bf1/_search

I've included a test as well.