title: wtf-form Email validation doesn't pass when email is optional
body: <p>I can't see what I'm doing wrong in a FlaskForm. I'd like the email address to be optional.</p>
  
  <p>Even if an entered email  is valid, it does not pass validation if email is optional.
  It might be my imagination but I think this used to work fine. I did update all my requirements recently, did something change? I'm now using:</p>
  
  <pre><code>Flask==1.0.2
  Flask-WTF==0.14.2
  WTForms==2.2.1
  </code></pre>
  
  <p>example:</p>
  
  <pre><code>from wtforms import StringField
  from wtforms.validators import DataRequired, Optional,  Email
  
  class ProfileForm2(FlaskForm):
      """
      Form for user to add or edit profile. Email optional
      """
      # this passes with valid email:
      email_req =StringField('Email', validators=[DataRequired(), Email()]) 
      # this never passes with same valid email
      email = StringField('Email (optional)', validators=[Optional(), Email()])
  </code></pre>
  
  <p>Any suggestions?</p>