Added a new '/files' endpoints test using authentication.
Added a new '/files' endpoints test using authentication.

A new test was added (FilesTest.AuthenticationTest) to probe the
`/files/*` endpoints' behavior when HTTP authentication is enabled.

Review: https://reviews.apache.org/r/45249/

zmalik/mesos,verizonlabs/mesos,reneploetz/mesos,verizonlabs/mesos,kaysoky/mesos,craimbert/mesos,abudnik/mesos,yuquanshan/customizedMesos,Aman-Jain-14/customizedMesos,chhsia0/mesos,reneploetz/mesos,abudnik/mesos,craimbert/mesos,Gilbert88/mesos,jpeach/mesos,chhsia0/mesos,zmalik/mesos,dforsyth/mesos,reneploetz/mesos,gsantovena/mesos,neilconway/mesos,Aman-Jain-14/customizedMesos,verizonlabs/mesos,Gilbert88/mesos,kaysoky/mesos,craimbert/mesos,dforsyth/mesos,verizonlabs/mesos,Aman-Jain-14/customizedMesos,verizonlabs/mesos,neilconway/mesos,reneploetz/mesos,zmalik/mesos,verizonlabs/mesos,gsantovena/mesos,jpeach/mesos,verizonlabs/mesos,dforsyth/mesos,Aman-Jain-14/customizedMesos,gsantovena/mesos,dforsyth/mesos,neilconway/mesos,gsantovena/mesos,asamerh4/mesos,jpeach/mesos,chhsia0/mesos,dforsyth/mesos,Gilbert88/mesos,zmalik/mesos,kaysoky/mesos,dforsyth/mesos,gsantovena/mesos,abudnik/mesos,Gilbert88/mesos,gsantovena/mesos,kaysoky/mesos,zmalik/mesos,andschwa/mesos,chhsia0/mesos,jpeach/mesos,craimbert/mesos,Gilbert88/mesos,chhsia0/mesos,reneploetz/mesos,craimbert/mesos,reneploetz/mesos,neilconway/mesos,asamerh4/mesos,asamerh4/mesos,kaysoky/mesos,zmalik/mesos,asamerh4/mesos,andschwa/mesos,abudnik/mesos,Aman-Jain-14/customizedMesos,yuquanshan/customizedMesos,neilconway/mesos,yuquanshan/customizedMesos,andschwa/mesos,asamerh4/mesos,chhsia0/mesos,chhsia0/mesos,andschwa/mesos,Aman-Jain-14/customizedMesos,jpeach/mesos,kaysoky/mesos,asamerh4/mesos,andschwa/mesos,dforsyth/mesos,kaysoky/mesos,abudnik/mesos,abudnik/mesos,zmalik/mesos,craimbert/mesos,craimbert/mesos,reneploetz/mesos,andschwa/mesos,andschwa/mesos,neilconway/mesos,Gilbert88/mesos,Aman-Jain-14/customizedMesos,yuquanshan/customizedMesos,yuquanshan/customizedMesos,asamerh4/mesos,yuquanshan/customizedMesos,jpeach/mesos,jpeach/mesos,Gilbert88/mesos,yuquanshan/customizedMesos,neilconway/mesos,abudnik/mesos,gsantovena/mesos