fix(storage): use correct host header

[coryan]

Some of our requests were using the wrong Host: header, luckily the
service is forgiving, and the testbench too.

Part of the work for #5072

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This change is 

[codecov]

Codecov Report

Merging #5085 into master will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #5085      +/-   ##
==========================================
- Coverage   95.82%   95.82%   -0.01%     
==========================================
  Files         985      986       +1     
  Lines       90210    90236      +26     
==========================================
+ Hits        86445    86469      +24     
- Misses       3765     3767       +2     

Impacted Files	Coverage Δ
.../cloud/pubsub/internal/subscription_batch_source.h	100.00% <ø> (ø)
...loud/pubsub/internal/subscription_message_source.h	100.00% <ø> (ø)
google/cloud/pubsub/publisher_options.h	100.00% <ø> (ø)
google/cloud/pubsub/topic_admin_client.h	100.00% <ø> (ø)
google/cloud/storage/internal/curl_client.h	100.00% <ø> (ø)
google/cloud/storage/internal/curl_client.cc	97.28% <100.00%> (+0.01%)	⬆️
google/cloud/storage/internal/curl_wrappers.cc	94.33% <100.00%> (+2.67%)	⬆️
...oogle/cloud/storage/internal/curl_wrappers_test.cc	100.00% <100.00%> (ø)
...bigtable/examples/bigtable_hello_instance_admin.cc	82.02% <0.00%> (-2.25%)	⬇️

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