title: Adding a non-nullable column to existing table fails. Is the "value" attribute being ignored?
body: <p>Background: we have a Grails 1.3.7 app and are using Liquibase to manage our database migrations.</p>
  
  <p>I am trying to add a new column to an existing table which is not empty.</p>
  
  <p>My changeset looks like this:</p>
  
  <pre><code>    changeSet(author: "someCoolGuy (generated)", id: "1326842592275-1") {
          addColumn(tableName: "layer") {
              column(name: "abstract_trimmed", type: "VARCHAR(455)", value: "No text") {
                  constraints(nullable: "false")
              }
          }
      }
  </code></pre>
  
  <p>Which should have inserted the value 'No text' into every existing row, and therefore satisfied the not null constraint. <a href="http://www.liquibase.org/manual/add_column" rel="noreferrer">Liquibase "Add Column" docs</a>.</p>
  
  <p>But when the migrations changesets are being applied I get the following exception:</p>
  
  <pre><code>liquibase.exception.DatabaseException: Error executing SQL ALTER TABLE layer ADD abstract_trimmed VARCHAR(455) NOT NULL: ERROR: column "abstract_trimmed" contains null values
  </code></pre>
  
  <p>Which looks to me like it is not using the 'value' attribute.</p>
  
  <p>If I change my changeset to work look like the following I can achieve the same thing. But I don't want to (and shouldn't have to) do this.</p>
  
  <pre><code>    changeSet(author: "someCoolGuy (generated)", id: "1326842592275-1") {
          addColumn(tableName: "layer") {
              column(name: "abstract_trimmed", type: "VARCHAR(455)")
          }
  
          addNotNullConstraint(tableName: "layer", columnName:"abstract_trimmed", defaultNullValue: "No text")
      }
  </code></pre>
  
  <p>Is Liquibase really ignoring my <code>value</code> attribute, or is there something else going on here that I can't see?</p>
  
  <p>I am using Grails 1.3.7, Database-migration plugin 1.0, Postgres 9.0</p>