Dataset.delete, Table.delete and Job.cancel ignore the if-non-match header

[mziccard]

It seems to me that the following apis

• Dataset.delete
• Table.delete
• Job.cancel

Ignore the HTTP if-none-match header (they fail as expected if if-match header does not match).

[garrettjonesgoogle]

Closing because #469 is being tracked on the backlog instead of in an open issue.