Fix from vs. continue

Recent MediaWiki versions (>= 1.19?) don't return `@apfrom` anymore. Instead, `@apcontinue` is returned in the response body.

This is inherently untestable with FakeMediaWiki and may not work on older MediaWiki versions. But testing against a live instance shows the effect:

````ruby
# before:
MediaWiki::Gateway.new('https://en.wikipedia.org/w/api.php').list('Media').size #=> 500

# after:
MediaWiki::Gateway.new('https://en.wikipedia.org/w/api.php').list('Media').size #=> 2373
````