accept string in assert.fileContent and noFileContent

With this PR it's possible to check the file content with a simple string.

This is more readable:
`assert.noFileContent('models/user.js', 'App.User = DS.Model.extend');`

than:
`assert.noFileContent('models/user.js', /App\.User = DS\.Model\.extend/);`

I made the change such that the previous behaviour still works :wink: