This commit fixes #251, which causes incorrect code to be generated when
there are multiple oneofs in the same message.

In #221, we added code to force int-sizing for oneof enums, but we only
set vars["opt_comma"] initially, before entering the loop over the
message's oneofs. This caused commas to be omitted when generating the
enums for subsequent oneofs after the first oneof.

This commit resets vars["opt_comma"] every time through the loop that
generates the enum declarations for the message's oneofs.