adding the FROM to postgreSQL UPDATE with inner joins related to #111

This fix is to include FROM to UPDATE statement and UpdateManager in order to support Updates that perform columnA = columnB like described here in a Rails issue https://github.com/rails/rails/issues/13496 and I was trying to fix with String manipulation here https://github.com/rails/rails/pull/15631

Also related to #111

I'd like to have some feedback to polish the code, thanks!

cc @rafaelfranca
this is not valid syntax in postgres; you want something like `UPDATE "users" SET "name" =...`