`const` declaration throwing in iOS safari browser

[mitch-mitchel]

const here:

can-ajax/can-ajax.js

Line 268 in c4e4c26

const result = o.beforeSend.call( o, xhr, o );

is throwing in iOS 9/safari with:
SyntaxError: Unexpected keyword 'const'. Const declarations are not supported in strict mode.

I will fix it in the mix, and do an insta PR to change this to var which fixes it locally for me.