title: Can't change border radius of text-field variant=filled in Material-ui-core
body: <h2>Current Behavior </h2>
  
  
  
  <p>my current component is as follows </p>
  
  <pre><code>export const BirthdayTextFields = styled(TextField)`
    width: 80px;
    margin-right: 10px;
    border-radius: 50px;
  `;
  </code></pre>
  
  <p>and I'm using it as follows: </p>
  
  <pre><code>&lt;BirthdayTextFields
                        id="filled-dense-hidden-label"
                        margin="dense"
                        hiddenLabel
                        variant="filled"
                        placeholder="18"
                        inputProps={{ 'aria-label': 'dense hidden label' }}
                        onChange={this.handleChange('day')}
                      /&gt;
  </code></pre>
  
  <p>since the variant="failed" it doesn't allow me to set the border-radius, I have tried overriding the rule fo TextField Component by following trick: </p>
  
  <pre><code>export const TextFieldWrapper = styled(TextField)`
    fieldset {
      border-radius: 50px;
    }
  `;
  </code></pre>
  
  <p>and I'm using like this, since it is variant=outlined the overriding works</p>
  
  <pre><code>&lt;TextFieldWrapper
                    id="outlined-dense"
                    label="Last name"
                    margin="dense"
                    variant="outlined"
                    onChange={this.handleChange('lastName')}
                  /&gt;
  </code></pre>
  
  <p>So how to solve this out? I have also try adding the fieldset value to BirthdayTextFields but it doesn't make any effect mostly becaues of the variant="filled"</p>
  
  <pre class="lang-js prettyprint-override"><code>export const BirthdayTextFields = styled(TextField)`
    width: 80px;
    margin-right: 10px;
    fieldset {
      border-radius: 50px;
    }
  `;
  </code></pre>
  
  <h2>Environment</h2>
  
  <p>Windows 10</p>
  
  <p>Tech : Version</p>
  
  <p>Material-UI:  ^3.9.3 </p>
  
  <p>React : ^16.8.6 </p>
  
  <p>Browser : Chrome </p>