support for long.js

[lishengguo]

writeLong(Long.fromValue(7))

[mention-bot]

@lishengguo, thanks for your PR! By analyzing the history of the files in this pull request, we identified @fengmk2, @gxcsoccer and @xusiyuan841028 to be potential reviewers.

[codecov]

Codecov Report

Merging #85 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master     #85   +/-   ##
======================================
  Coverage    96.1%   96.1%           
======================================
  Files           7       7           
  Lines        1079    1079           
  Branches      204     204           
======================================
  Hits         1037    1037           
  Misses         42      42

Impacted Files	Coverage Δ
lib/v2/encoder.js	94.57% <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 8e8cfc6...83f6531. Read the comment docs.

[fengmk2]

any test cases?

[lishengguo]

add test case : Long.fromNumber(0)

[fengmk2]

detect val is not equal string first.

if (typeof val !== 'number') {
  if (typeof val !== 'string') {
    val = val + '';
  }
  if (utility.isSafeNumberString(val)) {
    val = Number(val);
  }
}

[fengmk2]

Please rebase from master branch.