Fix attempting to access deferred fields in stdimage file descriptor

[chaosk]

StdImageField.set_variations will no longer attempt to access file field when the file field is deferred.

Before this change, any instance with deferred image field would result in an additional query to retrieve the image field value, as set_variations attempts to overwrite image field value.

Fixes #203

[codecov]

Codecov Report

Merging #204 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #204      +/-   ##
==========================================
+ Coverage   96.92%   96.94%   +0.01%     
==========================================
  Files           5        5              
  Lines         228      229       +1     
==========================================
+ Hits          221      222       +1     
  Misses          7        7

Impacted Files	Coverage Δ
stdimage/models.py	95.31% <100%> (+0.03%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1ff71ac...6ce9590. Read the comment docs.

[codingjoe]

Hi @chaosk good work. With tests and everything, love it!
I would have once request. Could you extend your test, that it ensures that you can access the deferred field and also the variations are accessible?

[codingjoe]

Released in 4.0.1

[codingjoe]

Thanks @chaosk what a pleasure!