Fix a bug in apitools for missing bodies on POSTs.
Fix a bug in apitools for missing bodies on POSTs.

It turns out that for some API calls (eg storage.v1.objects.copy), an empty
body is perfectly valid. Currently, we fail in this case with a terrible error
message.

(reviewed internally)

b-daniels/apitools,pcostell/apitools,thobrla/apitools,cherba/apitools,google/apitools,houglum/apitools,craigcitro/apitools,betamos/apitools,kevinli7/apitools