[BEAM-1502] GroupByKey should not return bare lists in DirectRunner.

[robertwb]

This leads to invalidated expectations on other runners.

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[robertwb]

R: @charlesccychen Yet another user was bitten by this.

[charlesccychen]

Thanks! LGTM.

CC: @mariapython

[coveralls]

Coverage increased (+0.01%) to 70.636% when pulling 61a3179 on robertwb:stream-iter into 7e4719c on apache:master.