title: How to validate disabled control using reactive forms (validation is not triggered)
body: <p>Let's say that I have this form structure: </p>
  
  <pre><code>      this.entryForm = this.formBuilder.group({
              date: [{value:'' , disabled: true}, [Validators.required]],
              notes: [''],
              sum_credit: [{value:'', disabled: true }],   
              sum_debit: [{value:'', disabled: true}],
              items: this.initItems()
            });
  // set validation function to sum_credit
  
     this.entryForm.controls['sum_credit'].setValidators([CommonValidations.validateSomthing(...)]);
  </code></pre>
  
  <p>The <code>sum_credit</code> is disabled because it's value is always calculated. 
  Now I need to validate that the <code>sum_credit</code> is equaled to <code>sum_debit</code>, and I'm already doing that using <code>validateSomthing</code> function.
  The problem is that the <code>validateSomthing</code> is not triggered because the control is disabled. How can I fix that? </p>
  
  <p>Thanks</p>