Fix nullifying other custom method decorators bug

[MichalLytek]

This PR changes the resolver's function calling mechanism.
Instead of calling the stored reference to original function (captured on decorator evaluation), it now calls the method on target instance - object[methodName].
This allows for the best interoperability with other custom decorators that overwrite the descriptor object to add some functionality.

Closes #166 🔒

[codecov]

Codecov Report

Merging #247 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #247   +/-   ##
=======================================
  Coverage   95.88%   95.88%           
=======================================
  Files          67       67           
  Lines        1045     1045           
  Branches      183      182    -1     
=======================================
  Hits         1002     1002           
  Misses         42       42           
  Partials        1        1

Impacted Files	Coverage Δ
src/decorators/Field.ts	100% <ø> (ø)	⬆️
src/decorators/FieldResolver.ts	93.33% <ø> (ø)	⬆️
src/resolvers/create.ts	100% <100%> (ø)	⬆️
src/helpers/resolver-metadata.ts	80% <100%> (ø)
src/decorators/Mutation.ts	100% <100%> (ø)	⬆️
src/decorators/Subscription.ts	100% <100%> (ø)	⬆️
src/decorators/Query.ts	100% <100%> (ø)	⬆️

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