cache: Fix Delete Handling

[dave-tucker]

We can't rely on the Delete field of RowUpdate2 being non-nil as
ovsdb-server sends a {"delete": null} which the Go JSON parser makes
equivalent to nil.

Signed-off-by: Dave Tucker dave@dtucker.co.uk

[coveralls]

Pull Request Test Coverage Report for Build 1334518988

• 7 of 7 (100.0%) changed or added relevant lines in 2 files are covered.
• 25 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.3%) to 73.004%

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Files with Coverage Reduction	New Missed Lines	%
cache/cache.go	3	71.76%
server/monitor.go	3	45.86%
client/client.go	19	68.39%

Totals
Change from base Build 1333653363:	-0.3%
Covered Lines:	4005
Relevant Lines:	5486

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💛 - Coveralls

[trozet]

/lgtm