Fixed utc.valueOf not taking DST into account

[SkaceKamen]

Use the current date to determine internal timezone offset instead of using a new Date instance, since the new Date returns a different offset based on whenever DST is in effect at the moment.

This PR fixes #1804

[codecov]

Codecov Report

Merging #1815 (b88a359) into dev (c7a3f73) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##               dev     #1815   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          180       180           
  Lines         2033      2034    +1     
  Branches       523       524    +1     
=========================================
+ Hits          2033      2034    +1     

Impacted Files	Coverage Δ
src/plugin/utc/index.js	100.00% <100.00%> (ø)
src/locale/gl.js	100.00% <0.00%> (ø)
src/locale/ku.js	100.00% <0.00%> (ø)

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[xvaara]

this is same as #1448
but since it's a year old and not merged (even though it's a big problem), one has to wonder where to focus of this project is.

[iamkun]

Thanks, fixed in #1448