title: Alembic can't drop server_default
body: <p>I'm trying to add new column to existing table, but getting error when removing server_default.</p>
  
  <pre><code>from alembic import op
  import sqlalchemy as sa
  
  
  # revision identifiers, used by Alembic.
  revision = 'a29663449f3c'
  down_revision = 'e42f9a7ca3d2'
  branch_labels = None
  depends_on = None
  
  
  def upgrade():
      # ### commands auto generated by Alembic - please adjust! ###
      op.add_column('register_code', sa.Column('interest', sa.Integer(), nullable=False, server_default='0'))
      op.alter_column('register_code', 'interest', server_default=None)
      # ### end Alembic commands ###
  
  
  def downgrade():
      # ### commands auto generated by Alembic - please adjust! ###
      op.drop_column('register_code', 'interest')
      # ### end Alembic commands ###
  </code></pre>
  
  <p>Getting next error when execute <code>flask db migrate</code></p>
  
  <pre><code>sqlalchemy.exc.OperationalError: (sqlite3.OperationalError) near "ALTER": syntax error
  [SQL: ALTER TABLE register_code ALTER COLUMN interest DROP DEFAULT]
  (Background on this error at: http://sqlalche.me/e/e3q8)
  </code></pre>
  
  <p>Maybe SQLLite not support dropping server_default?</p>