BUG: PEBKAC bug in Float64Index
closes #7066

@jreback ok with this? passed on my travis 
 can I add a tests of a 2 elent index that has a nan and a value and uses isin (and then check that the isin values is in and another when it's False)
 you add (typo)!
 I saw u r giving. talk on Pandas!
cool - u have slides? what r u talking on?
 Topic is all the awesome features of pandas since v0.11. IPython notebook in the works. 
 sounds awesome!

will see if I can make it
 That'd be great!
 @jreback good 2 go? test added
 looks good
