fix(rest): missing user-agent separator

[coryan]

This change is 

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (ea2b632) 93.80% compared to head (49a7682) 93.81%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #11473   +/-   ##
=======================================
  Coverage   93.80%   93.81%           
=======================================
  Files        1807     1807           
  Lines      163118   163120    +2     
=======================================
+ Hits       153020   153026    +6     
+ Misses      10098    10094    -4     

Impacted Files	Coverage Δ
google/cloud/internal/curl_impl.cc	90.62% <100.00%> (+0.02%)	⬆️
...loud/internal/curl_rest_client_integration_test.cc	98.80% <100.00%> (+<0.01%)	⬆️
...oud/storage/tests/curl_request_integration_test.cc	97.30% <100.00%> (ø)

... and 5 files with indirect coverage changes

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[dbolduc]

comment: I don't know what gives nicer errors between AllOf(Contains(m1), Contains(m2), Contains(m3)) and IsSupersetOf({m1, m2, m3})

[coryan]

I did not want to fiddle with the type of {"foo", "bar", StartsWith("baz")}, or {Eq("foo"), Eq("bar"), StartsWith("baz")}. I think both are ambiguous (they need to be initializer_list<T> and T can be too many things.