title: @submit.prevent not working on vue md-input
body: <p>when I click 'login button' even I don't fill data in md-input it still running, 
  I test my onSubmit() method by login with my user and it works!
  I don't think I do thing wrong in the method so I guess that my form is incorrect.
  here is my code : </p>
  
  <p><strong>my form</strong></p>
  
  <pre><code> &lt;form @submit.prevent="onSubmit"&gt;
      &lt;login-card header-color="green"&gt;
        &lt;h4 slot="title" class="title"&gt;CCRP Sign On&lt;/h4&gt;
        &lt;p slot="description" class="description"&gt;IT solution by เจ้เก๋ IT-PM&lt;/p&gt;
        &lt;md-field class="md-form-group" slot="inputs"&gt;
          &lt;md-icon&gt;account_box&lt;/md-icon&gt;
          &lt;label&gt;ID...&lt;/label&gt;
          &lt;md-input v-model.trim="userId" type="text"&gt;&lt;/md-input&gt;
        &lt;/md-field&gt;
        &lt;md-field class="md-form-group" slot="inputs"&gt;
          &lt;md-icon&gt;lock_outline&lt;/md-icon&gt;
          &lt;label&gt;Password...&lt;/label&gt;
          &lt;md-input v-model.trim="password" type="password"&gt;&lt;/md-input&gt;
        &lt;/md-field&gt;
        &lt;md-field class="md-form-group"&gt;
          &lt;md-icon&gt;announcement&lt;/md-icon&gt;
          &lt;label&gt;Password...&lt;/label&gt;
        &lt;/md-field&gt;
        &lt;md-button slot="footer" class="md-simple md-success md-lg" type="submit"&gt;Login&lt;/md-button&gt;
      &lt;/login-card&gt;
    &lt;/form&gt;
  </code></pre>
  
  <p><strong>in scrpit methods</strong></p>
  
  <pre><code>async onSubmit() {
        const authData = {
          userId: this.userId,
          password: this.password
        };
        await this.login(authData).then(() =&gt; {
          if (this.isAuthenticated) {
            this.$router.push("dashboard");
          } else {
            console.log("err");
          }
        });
      },
  </code></pre>
  
  <p>can you help me solve this?</p>