Protect against {boolean}.to_i raising on invalid stale-while-revalidate cache control directives

[matthutchinson]

Invalid cache control directives for stale-while-revalidate are raising here when we attempt to parse the integer value.

Eg. an invalid cache-control directive such as this

public, max-age=60, stale-while-revalidate

The gem converts directives without values to true here.

This PR will allow the gem to ignore these invalid values (see spec), and prevent this error from raising;

[NoMethodError] undefined method 'to_i' for true

Added tests to verify behaviour.