Remove "fallback src" in srcset generation

[jayeb]

Describe the bug
We're currently adding a "sizeless" source URL to the end of our srcset attributes as a "fallback source", which is incorrect. The "fallback source" for an image element using the srcset attribute should be provided in the src attribute, not at the end of the srcset list.

See the lines in question here: https://github.com/imgix/react-imgix/blob/master/src/react-imgix.js#L124-L125

To Reproduce
N/A

Expected behaviour
Every item in the srcset list should include a size descriptor, which in our case is always a width value or a resolution multiplier.

Information:

• react-imgix version: >= 8.0.0

Additional context
This issue is also present in imgix-rails, and is being fixed here: imgix/imgix-rails#69

[frederickfogerty]

I've managed to sneak this into #224.

[frederickfogerty]

Fixed by #224

[frederickfogerty]

Released in 8.4.0 🎉