text: Title: Server default not matching for `func.now()` with SQLite
  Body: My model has 
  ```python
      created = Column(DateTime, server_default=func.now())
  ```
  And every time I invoke `autogenerate` with `compare_server_default=True` on sqlite, it generates the same migrations that do nothing:
  
  ```python
  def upgrade():
      # ### commands auto generated by Alembic - please adjust! ###
      with op.batch_alter_table('visitors', schema=None) as batch_op:
          batch_op.alter_column('created',
                 existing_type=sa.DATETIME(),
                 server_default=sa.text('(CURRENT_TIMESTAMP)'),
                 existing_nullable=True)
  
      # ### end Alembic commands ###
  
  
  def downgrade():
      # ### commands auto generated by Alembic - please adjust! ###
      with op.batch_alter_table('visitors', schema=None) as batch_op:
          batch_op.alter_column('created',
                 existing_type=sa.DATETIME(),
                 server_default=sa.text('(CURRENT_TIMESTAMP)'),
                 existing_nullable=True)
  
      # ### end Alembic commands ###
  ```