Include plus_code in PlaceDetailsRequest.FieldMask

[Jasper-M]

Without it you can't request a subset of fields in a details request that includes plus_code.

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[coveralls]

Coverage increased (+0.01%) to 73.854% when pulling 3cf87ce on Jasper-M:patch-1 into 08fed9a on googlemaps:master.

[Jasper-M]

@googlebot I signed it!

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[amuramoto]

Thanks for the PR!