fix the bug of the return value of conn.WriteTo

[JemmyH]

Change-Id: I04c0e0b0d4ef9502195b38a545f04c9f1bc305af

---

name: Pull request
about: Bug fix
title: 'fix the bug of the return value of conn.WriteTo'
labels: ''
assignees: ''

1. Are you opening this pull request for bug-fixes, optimizations or new feature?

bug-fixs

2. Please describe how these code changes achieve your intention.

conn.WriteTo should return the number of bytes actually written

3. Please link to the relevant issues (if any).

#343

4. Which documentation changes (if any) need to be made/updated because of this PR?

connection.go

4. Checklist

• I have squashed all insignificant commits.
• I have commented my code for explaining package types, values, functions, and non-obvious lines.
• I have written unit tests and verified that all tests passes (if needed).
• I have documented feature info on the README (only when this PR is adding a new feature).
• (optional) I am willing to help maintain this change if there are issues with it later.

[codecov]

Codecov Report

Merging #344 (da19a3a) into dev (27667f3) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##              dev     #344   +/-   ##
=======================================
  Coverage   72.16%   72.16%           
=======================================
  Files          13       13           
  Lines        1376     1376           
=======================================
  Hits          993      993           
  Misses        315      315           
  Partials       68       68           

Flag	Coverage Δ
unittests	72.16% <0.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
connection.go	69.73% <0.00%> (ø)

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[panjf2000]

Thanks~