fix: create correct v4 signed url with cname

[jkwlui]

#859

This PR fixes the following:

1. Fix host:[cname] instead of host:storage.googleapis.com when cname is provided when building canonical request.
2. Use only the object's name in the request path when cname is used:

// Before
GET
/mycname.com/object_name

host:storage.googleapis.com

// After
GET
/object_name

host:mycname.com

[codecov]

Codecov Report

Merging #868 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #868      +/-   ##
==========================================
+ Coverage   95.26%   95.26%   +<.01%     
==========================================
  Files          11       11              
  Lines        1203     1205       +2     
  Branches      298      300       +2     
==========================================
+ Hits         1146     1148       +2     
  Misses         29       29              
  Partials       28       28

Impacted Files	Coverage Δ
src/file.ts	98.16% <100%> (ø)	⬆️

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