Add DateTime::to_utc

[pitdicker]

To have an easy way to convert any DateTime<Tz> to DateTime<Utc>.

As discussed in #1318.

[codecov]

Codecov Report

Merging #1325 (5383737) into 0.4.x (0032431) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##            0.4.x    #1325   +/-   ##
=======================================
  Coverage   91.25%   91.26%           
=======================================
  Files          38       38           
  Lines       17164    17174   +10     
=======================================
+ Hits        15663    15673   +10     
  Misses       1501     1501           

Files	Coverage Δ
src/datetime/mod.rs	85.03% <100.00%> (+0.06%)	⬆️
src/datetime/tests.rs	96.52% <100.00%> (+0.01%)	⬆️

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[djc]

In my mind the proposed behavior was to adjust the DateTime for the offset, not to just discard the offset.

[pitdicker]

The value is already stored in UTC, so it does nothing special compared to into().