[update] bindplan sticked with a columnfilter

[hsinhoyeh]

This patch sticked a updatePlan to a column filter. So if you used two different column filters for update, it will store two updatePlan into DbMap. But that would be a problem to your design if you use thousands of column filters, which is unlikely happened.

Reason: when updating specific columns of a table, the current approach sticks all column filters to a single bindPlan, which makes it only work for the first one. However, generating updatePlan on-the-fly sounds inefficient, instead sticking a column filter to it's own updatePlan sounds like a good trade-off.

related issues: #308, #325

[coveralls]

Coverage increased (+0.5%) to 67.093% when pulling 9e3b102 on hsinhoyeh:master into 88f3ff2 on go-gorp:master.