fix(typescript): cast Object.assign value to ServiceOptions

[bcoe]

Fixes #481

[codecov]

Codecov Report

Merging #483 into master will decrease coverage by 16.99%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           master     #483       +/-   ##
===========================================
- Coverage   96.01%   79.01%   -17.00%     
===========================================
  Files          21       21               
  Lines        3112     3112               
  Branches      246      224       -22     
===========================================
- Hits         2988     2459      -529     
- Misses        123      651      +528     
- Partials        1        2        +1     

Impacted Files	Coverage Δ
src/google-apis/auth-client.ts	71.00% <100.00%> (-21.85%)	⬇️
src/interfaces/message-builder.ts	0.00% <0.00%> (-100.00%)	⬇️
src/index.ts	0.00% <0.00%> (-98.71%)	⬇️
utils/errors-api-transport.ts	0.00% <0.00%> (-91.36%)	⬇️
src/interfaces/koa2.ts	0.00% <0.00%> (-78.88%)	⬇️
src/interfaces/koa.ts	0.00% <0.00%> (-71.43%)	⬇️
src/interfaces/manual.ts	95.88% <0.00%> (-4.12%)	⬇️
src/logger.ts	87.35% <0.00%> (-2.30%)	⬇️
src/configuration.ts	96.23% <0.00%> (-1.26%)	⬇️

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