Issue type:

[ ] question
[x] bug report
[ ] feature request
[ ] documentation issue

Database system/driver:

[x] all

TypeORM version:

[x] latest
[ ] @next
[ ] 0.x.x (or put your version here)

Steps to reproduce or a small repository showing the problem:
If you add a transformer to a column which serializes the property value to the desired format, this transformation is not considered in SubjectChangedColumnsComputer and the column is always marked as changed if you are working with a complex object type for the column which is serialized to string for example.

export interface MyComplexType {
  foo: string;
  bar: number;
  xy: Date;
}

export const myTransformer: ValueTransformer = {
  from: dbValue => {
    const parsed = (dbValue || '').split(',');
    return { foo: parsed[0], bar: +parsed[1] || undefined };
  },
  to: entityValue => `${entityValue.foo},${entityValue.bar}`
};

@Entity('foo')
export class Foo {
  @PrimaryGeneratedColumn({ type: 'int' })
  readonly id: number;

  @Column('text', { transformer: myTransformer })
  transformed: MyComplexType;
}

Why is column.getEntityValue() not called with the transform flag to true in computeDiffColumns of SubjectChangedColumnsComputer?