[BEAM-2636] Make sure we only override the correct class for WriteToBigQuery

[sb2nov]

Follow this checklist to help us incorporate your contribution quickly and easily:

• Make sure there is a JIRA issue filed for the change (usually before you start working on it). Trivial changes like typos do not require a JIRA issue. Your pull request should address just this issue, without pulling in other changes.
• Each commit in the pull request should have a meaningful subject line and body.
• Format the pull request title like [BEAM-XXX] Fixes bug in ApproximateQuantiles, where you replace BEAM-XXX with the appropriate JIRA issue.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Run mvn clean verify to make sure basic checks pass. A more thorough check will be performed on your pull request automatically.
• If this contribution is large, please file an Apache Individual Contributor License Agreement.

---

R: @aaltay PTAL

[aaltay]

LGTM. I will merge after tests pass.

Please send a CP PR, if there will be an RC3 for the current release.

[coveralls]

Coverage increased (+0.003%) to 70.627% when pulling 9056783 on sb2nov:BEAM-2636-bq-override into a6f460f on apache:master.

[sb2nov]

Thanks will do the cherry pick if there is a new RC