cloud/bigtable: Uses simple string concatentation to build ReadItem.Column
cloud/bigtable: Uses simple string concatentation to build ReadItem.Column

Change-Id: I6a568f01728cbaeafbbbfdd6798ad9a219424b83
Reviewed-on: https://code-review.googlesource.com/2930
Reviewed-by: Chris Broadfoot <0c2851a4f8ab4aa5606e69c0757771dd13dbe686@google.com>

michael-grosshans/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,michael-grosshans/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go