Fix how the key regexp for unions is calculated
Fix how the key regexp for unions is calculated

Union would produce spurious empty keys, for example in the expression
  [ key /a/ . ([ l1 ] | [ l2 ]) ]
the union would get an atype of '//' which is wrong, since the subtree []
does not contribute to an atype.

lutter/augeas,GeoffWilliams/augeas,domcleal/augeas,jasperla/augeas,hercules-team/augeas-do-not-use,mlichvar/augeas,ptoscano/augeas,lutter/augeas,camptocamp/augeas,jjlin/augeas,kunkku/augeas,bkearney/augeas,MikaelSmith/augeas,pevalme/augeas,camptocamp/augeas,dafugg/augeas,kumy/augeas,bkearney/augeas,bkearney/augeas,manandbytes/augeas,kumy/augeas,ptoscano/augeas,dafugg/augeas,mlichvar/augeas,pevalme/augeas,bkearney/augeas,jtopjian/augeas,camptocamp/augeas,domcleal/augeas,hercules-team/augeas-do-not-use,jjlin/augeas,hercules-team/augeas,jjlin/augeas,kumy/augeas,ptoscano/augeas,mchf/augeas,pevalme/augeas,raphink/augeas,lutter/augeas,raphink/augeas,kunkku/augeas,manandbytes/augeas,MikaelSmith/augeas,camptocamp/augeas,mlichvar/augeas,hercules-team/augeas,hercules-team/augeas-do-not-use,mchf/augeas,kumy/augeas,GeoffWilliams/augeas,jasperla/augeas,jtopjian/augeas,domcleal/augeas,kunkku/augeas,dafugg/augeas