fix(apimock): toISOString support on dates in IE8
fix(apimock): toISOString support on dates in IE8

We used Date.prototype.toISOString(), which wasn’t supported on IE8.

Reference:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global
_Objects/Date/toISOString

Fixes #51

seriema/angular-apimock,seriema/angular-apimock