fix DONT_FRAGMENT error when using a IPv6 connection on Windows

[marten-seemann]

The syscall setting the IP_DONTFRAGMENT socket option will fail if the underlying connection is a IPv6 connection. Since Go doesn't allow us to distinguish between IPv6 and dual-stack connections, we can only ignore the error here.

[codecov]

Codecov Report

Merging #3178 (22a703b) into master (50746db) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3178      +/-   ##
==========================================
+ Coverage   85.35%   85.37%   +0.01%     
==========================================
  Files         133      133              
  Lines        9824     9819       -5     
==========================================
- Hits         8385     8382       -3     
+ Misses       1063     1062       -1     
+ Partials      376      375       -1     

Impacted Files	Coverage Δ
conn_windows.go	56.52% <100.00%> (-0.62%)	⬇️

---

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 50746db...22a703b. Read the comment docs.