make DialEarly return EarlySession

[l2dy]

Fixes #2619.

[codecov]

Codecov Report

Merging #2621 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2621   +/-   ##
=======================================
  Coverage   86.21%   86.21%           
=======================================
  Files         122      122           
  Lines        9777     9777           
=======================================
  Hits         8429     8429           
  Misses       1004     1004           
  Partials      344      344           

Impacted Files	Coverage Δ
client.go	78.29% <100.00%> (ø)

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[marten-seemann]

Thank you!