gensupport: fix race in multipartReader.Close
gensupport: fix race in multipartReader.Close

If multiple threads call Close, they race on the pipeOpen field.

The race detector actually caught this.

Fixes #272.

Change-Id: Iab58d433752dda515d5b73d5c7ee899a30b2eec4
Reviewed-on: https://code-review.googlesource.com/8830
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

googleapis/google-api-go-client,google/google-api-go-client,google/google-api-go-client,googleapis/google-api-go-client