Image true size doesn't respect size attributes

[joehoyle]

Steps to reproduce:

1. Add a new image via Gutenberg
2. Type in a custom size for the image. Here's one I made earlier:
3. Save the post

Here's the image on the front-end:

The image attributes are outputted on the front-end, but it seems the image size (from tachyon) is using the image-size class from the image in precedence of the width/height attributes. This appears to mean it's not possible to insert an image (native size) into a page that isn't a registered image size. Given we can do that due to Tachyon, it seems like this should be supported.

Not sure how this is meant to work with responsive images though

I'd expect to see the native image size (downloaded file) matching the size that I had entered in Gutenberg.

[roborourke]

I think this is due to how it's figured out in Tachyon, with the size class taking precedence so might need to fix it there. To be honest it could be worth refactoring the whole Tachyon singleton class now that we have tests, it's a messy beast.

[joehoyle]

@roborourke I'd suggest we just switch around that logic for now 😬