Add support for dynamic serializer fields

[melvinkcx]

Replace serializer.get_field() with serializer.fields to support dynamic fields.

fix #108

[browniebroke]

Do you think you could write a test to cover the dynamic fields case? Thanks!

[melvinkcx]

sure, I added a test case. Please check again :)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ
drf_excel/renderers.py	86.74% <100.00%> (ø)

[browniebroke]

Thanks for the fix!