title: Setting default value after initialization in SelectField flask-WTForms
body: <p>I would like to pre-select the value for <code>SelectField</code> when displaying it to user. <code>default</code> argument works when it is passed at the time of instantiation but does not work once the field is initialized.</p>
  
  <pre><code>class AddressForm(Form):
      country = SelectField('Country',choices=[('GB', 'Great Britan'), ('US', 'United States')], default='GB')    # works
  </code></pre>
  
  <p>When I try to use <code>default</code> value to preselect option before presenting the form to user for editing, it doesn't work.</p>
  
  <pre><code>address_form = AddressForm()
  address_form.country.default='US'    # doesnot work
  </code></pre>
  
  <p>Need a solution to set default value to pre-set values before presenting to the user.</p>
  
  <p>Scenario 2: Also does not work </p>
  
  <pre><code>class AddressForm(Form):
          country = SelectField('Country')    # works
  
  address_form = AddressForm()
  address_form.country.choices=[('GB', 'Great Britan'), ('US', 'United States')]
  address_form.country.default='US'    # doesnot work
  </code></pre>