Add Class SetThumbnailRequest

[daigo424]

Fixes: #606

Google::Apis::Core::HttpCommand.body was nil.

before:

Upload complete
Sending upload start command to https://www.googleapis.com/upload/youtube/v3/thumbnails/set?
Upload status final
Error - #<Google::Apis::ClientError: required: Required parameter: videoId>

after:

Sending upload command to https://www.googleapis.com/upload/youtube/v3/thumbnails/set?videoId=55XDuvqq1wI&upload_id=AEnB2UqGyNrZjrGaJYvfYEi3POef6dzvNqjYwSAH7zgYRmBa8Uvqkx6nq3g1h2SWvUq9ef8eq4cFj5gIZXMX7WkGe_qE9CxKFA&upload_protocol=resumable
Upload status final
Success - #<Google::Apis::YoutubeV3::SetThumbnailResponse:0x00007f90a0f4f320

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[googlebot]

CLAs look good, thanks!

[coveralls]

Coverage remained the same at 95.997% when pulling 13cac89 on daigoishii:add_class_set_thumbnail_request into 1210eab on google:master.

[meanphil]

Can someone please review and merge this?

[hoangzinh]

Please review this PR

[dazuma]

Unfortunately, this is automatically generated code. If we merge this, the automation will revert it the next time it runs. I'm investigating why the code generation is omitting this...

[grant]

Unfortunately, this is automatically generated code. If we merge this, the automation will revert it the next time it runs. I'm investigating why the code generation is omitting this...

As mentioned here, we can't merge this PR. Continuing tracking the issue with #606.