More validation for update item

[Ajaya Agrawal]

The schema looks like:
https://gist.github.com/anonymous/985b2c74f86d3544bd3b
The put request looks like:
https://gist.github.com/anonymous/ebd248f016fb1fa2bc04

We don't allow to modify primary key fields in a update item. So ideally the primary key fields should not be allowed at all in 'attribute_updates' field. Even if they are allowed there should be a check whether they are the same as the values in 'key' field. I would call it a logic bug also the reason of which is clear from the second query run by C*. Had there been a row with key as (b,1), it would be modified instead of the key queried for.

The first C* query run is:
SELECT * FROM "u_default_tenant"."u_fc38f99aad3911e4b9a49cd21e20b135" WHERE "u_Author"='a' AND "u_MessageId"='1'

The second C* query run is:

UPDATE "u_default_tenant"."u_fc38f99aad3911e4b9a49cd21e20b135" SET "u_Subject"='Vacation2',dyn_attr_dat=dyn_attr_dat{'aj':0x22626a22},dyn_attr_typ=dyn_attr_typ+{'aj':'S'},attr_exist=attr_exist+{'Author':1,'aj':1,'Subject':1,'MessageId':1} WHERE "u_Author"='b' AND "u_MessageId"='1' IF "u_Body"='Some vacation
issues' AND dyn_attr_dat['aj']=0x22626a22 AND dyn_attr_dat['name']=0x22616a22 AND "u_Subject"='Vacation2' AND "u_DateTime"=20140313164951", cons
istency=QUORUM