The SPA fallback option should always be a string, if the user configuration was specifed as boolean true it should have been replaced by the default string value (currently 404.html) in lib/common/options.js
Added test on empty string
Added missing test for spa-fallback.html
* fix: appveyor test failure
* ci: bring back yarn cache
* ci: turn off matrix
* refactor: use babel instead of esm in jest
* refactor: use es modules in fixtures