Ensure that assert_that takes a PCollection as its first argument.

[robertwb]

This can avoid (silent) error such as

with test_pipeline.TestPipeline() as p:
  bad_value = "Create0" >> beam.Create([1])  # Note missing "p | "
  beam_util.assert_that(bad_value, beam_util.equal_to([0]))

Be sure to do all of the following to help us incorporate your contribution
quickly and easily:

• Make sure the PR title is formatted like:
  [BEAM-<Jira issue #>] Description of pull request
• Make sure tests pass via mvn clean verify. (Even better, enable
  Travis-CI on your fork and ensure the whole test matrix passes).
• Replace <Jira issue #> in the title with the actual Jira issue
  number, if there is one.
• If this contribution is large, please file an Apache
  Individual Contributor License Agreement.

---

[coveralls]

Coverage decreased (-0.003%) to 70.165% when pulling 479cc01 on robertwb:assert-that into ef256df on apache:master.

[asfbot]

Refer to this link for build results (access rights to CI server needed):
https://builds.apache.org/job/beam_PreCommit_Java_MavenInstall/8292/

Failed Tests: 1

beam_PreCommit_Java_MavenInstall/org.apache.beam:beam-runners-spark: 1

• org.apache.beam.runners.spark.translation.streaming.ResumeFromCheckpointStreamingTest.testWithResume

--none--

[robertwb]

retest this please

[coveralls]

Coverage increased (+0.02%) to 70.193% when pulling 479cc01 on robertwb:assert-that into ef256df on apache:master.

[asfbot]

Refer to this link for build results (access rights to CI server needed):
https://builds.apache.org/job/beam_PreCommit_Java_MavenInstall/8491/

Failed Tests: 1

beam_PreCommit_Java_MavenInstall/org.apache.beam:beam-runners-spark: 1

• org.apache.beam.runners.spark.translation.streaming.ResumeFromCheckpointStreamingTest.testWithResume

--none--

[davorbonaci]

R: @aaltay

[aaltay]

LGTM