Don't output key for write only relations

[peakwinter]

If the write_only key is True on a relationship, its key is still included in the response body with an empty resource object ({'data': null}).

This was fixed in #230 for normal fields but not for relations.

[codecov-io]

Codecov Report

Merging #421 into master will decrease coverage by 0.02%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master     #421      +/-   ##
==========================================
- Coverage    93.1%   93.07%   -0.03%     
==========================================
  Files          54       54              
  Lines        3000     3002       +2     
==========================================
+ Hits         2793     2794       +1     
- Misses        207      208       +1

Impacted Files	Coverage Δ
rest_framework_json_api/renderers.py	85.76% <50%> (-0.24%)	⬇️

---

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 3bfff93...5dd8106. Read the comment docs.

[sliverc]

Thanks for your contribution. Looks good to me but could you write a test case for this? Thanks.

[sliverc]

Closing this PR in favor of #522