add interpolation variable matcher

here's something I'd love to use in devise-i18n 

based on something similar I made for loomio.
Please feel free to get me to rework anything that is unclear/ verbose 
In this feature, I am declaring is a pass :white_check_mark:  

A) the test locale string completely is empty / blank
*OR*
B) the test locale and the default locale have exactly the same interpolation variables

Is this ok?