title: SQLAlchemy, there is not history of deleted state of JSONB field
body: <p>For example I have a model:</p>
  
  <pre><code>class AttributeValue(db.Model):
      id = Column(Integer, primary_key=True)
      value = Column(JSONB(), nullable=False)
      attribute_id = Column(Integer, ForeignKey("attributes.id"), nullable=False)
      ...
  
      __tablename__ = 'attribute_value'
  </code></pre>
  
  <p>And there are records in the database for this table.
  If I change <strong><em>attribute_id</em></strong> of some record and try to get the History object for attribute_id field of this record, then I will get something like this </p>
  
  <pre><code>History(added=[2], unchanged=(), deleted=[1])
  </code></pre>
  
  <p>If I change <strong><em>value</em></strong> and try to get History object for this field, then I will get something like this</p>
  
  <pre><code>History(added=[{u'keywords': u'test'}], unchanged=(), deleted=())
  </code></pre>
  
  <p>So, after any changes value of JSONB field, that contains dict, History object doesn't know deleted value.
  This problem is also relevant for the JSON field.</p>
  
  <p>Does any know what is the problem?</p>