fix: fix the constructor of observable class is invalid

[BlackGanglion]

When observable class has constructor to init，should pass arguments into target.

[codecov]

Codecov Report

Merging #2 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master       #2   +/-   ##
=======================================
  Coverage   81.96%   81.96%           
=======================================
  Files          11       11           
  Lines         621      621           
  Branches      102      102           
=======================================
  Hits          509      509           
  Misses        112      112

Impacted Files	Coverage Δ
src/observer.ts	91.03% <100%> (ø)	⬆️

---

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 7fc04db...af0a801. Read the comment docs.