Reduce false positive by ending at an operator for issue #20

[cashlo]

This reduce false positive like

"update" eq $foo;
$foo eq "update" && $bar;
$foo ne "select" || $bar;

[coveralls]

Coverage increased (+0.1%) to 97.203% when pulling 12ef1b8 on cashlo:master into 13fa7f0 on guillaumeaubert:master.

[cashlo]

I was going to use qw{ . .= , => } as the list of operators because of usage like my $sql = sprintf "SELECT %s FROM %s" => $column, $table; but decided to align with current behaviour.

[oalders]

This breaks a couple of tests, which apparently pass under 5.10 on Travis.

#   Failed test 'ValuesAndExpressions::PreventSQLInjection - line 283 - Partial use of quote() and quote_identifier().'
#   at t/30-policy.t line 22.

#   Failed test 'ValuesAndExpressions::PreventSQLInjection - line 402 - Custom quoting method, ensure that defaults are disabled.'
#   at t/30-policy.t line 22.
# Expected 1 violations, got 0.