preview_image_url should handle unspecified capture size with options more gracefully

[rahim]

Currently this happens:

2.2.2 (main):0 > Litmus::Instant.preview_image_url(email_guid, "OL2010", images: "blocked")
=> "https://OL2010.instant-api.litmus.com/v1/emails/755d1f9f-ad28-460f-8e45-632e0eceab32/previews/OL2010/{:images=>\"blocked\"}"

This is mostly a consequence of supporting 1.9.x rather than using 2.x keyword args.

We can maintain 1.9.x support by detecting whether the third arg (capture_size) is a Hash or string, if a Hash assume it's the options, and use the default capture_size.

Alternatively (given we're yet to release) we could change the method signature and move capture_size in to the options hash too.