refactor(url_base): copy operation uses reference to underlying implementation

[alandefreitas]

The copy operation from a url_view_base should use the pointed implementation rather than the underlying implementation so that url_views that refer to other urls are accounted for.

fix #872

[cppalliance-bot]

Antora version: an automated preview of the documentation is available at https://873.urlantora.prtest2.cppalliance.org/site/index.html

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.20%. Comparing base (2e58683) to head (dddea38).
Report is 3 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #873      +/-   ##
===========================================
+ Coverage    99.19%   99.20%   +0.01%     
===========================================
  Files          157      157              
  Lines         8415     8415              
===========================================
+ Hits          8347     8348       +1     
+ Misses          68       67       -1     

Files with missing lines	Coverage Δ
include/boost/url/url_base.hpp	100.00% <ø> (ø)
src/url_base.cpp	99.63% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[cppalliance-bot]

GCOVR code coverage report https://873.url.prtest2.cppalliance.org/gcovr/index.html
LCOV code coverage report https://873.url.prtest2.cppalliance.org/genhtml/index.html
Coverage Diff https://873.url.prtest2.cppalliance.org/gcovr/coverage_diff.txt