Added support for `INSERT INTO foo SET bar=1234, foo=5678`

v2 branch already had support for 
    INSERT INTO foo ( a, b, c) VALUES ( 123, 456, 789 )
I've added support for this format
    INSERT INTO foo SET bar=1234, foo=5678
Not sure if I've done it in the best way though, it feels a bit clumsy. Any feedback?