Release gax-java v1.38.0

[andreamlin]

This pull request was generated using releasetool.

02-01-2019 12:02 PST

Implementation Changes

• Use ManagedChannelBuilder so user creds don't get overwritten (#650)
• Always add Content-Length header (#648)
• Use GoogleDefaultChannelBuilder (#627)

Internal / Testing Changes

• Add another unit test for JSON fieldmask serialization (#654)
• @AutoValue HttpRequestRunnable (#509)

[codecov]

Codecov Report

Merging #655 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #655   +/-   ##
=========================================
  Coverage     75.57%   75.57%           
  Complexity     1008     1008           
=========================================
  Files           187      187           
  Lines          4365     4365           
  Branches        343      343           
=========================================
  Hits           3299     3299           
  Misses          911      911           
  Partials        155      155

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[andreamlin]

PTAL

[andreamlin]

Release is at https://github.com/googleapis/gax-java/releases/tag/v1.38.0