fix: don't override custom pattern (#500)

src/vaadin-email-field.html

@@ -89,7 +89,7 @@
         /** @protected */
         _createConstraintsObserver() {
           // NOTE: pattern needs to be set before constraints observer is initialized
-          this.pattern = '^[a-zA-Z0-9.!#$%&’*+/=?^_`{|}~-]+@[a-zA-Z0-9-]+(?:\.[a-zA-Z0-9-]+)*$';
+          this.pattern = this.pattern || '^[a-zA-Z0-9.!#$%&’*+/=?^_`{|}~-]+@[a-zA-Z0-9-]+(?:\.[a-zA-Z0-9-]+)*$';
 
           super._createConstraintsObserver();
         }

test/email-field.html

@@ -18,6 +18,12 @@
     </template>
   </test-fixture>
 
+  <test-fixture id="custom-pattern">
+    <template>
+      <vaadin-email-field pattern=".+@example\.com"></vaadin-email-field>
+    </template>
+  </test-fixture>
+
   <script>
     const validAddresses = [
       '[email protected]',
@@ -59,6 +65,10 @@
         expect(input.type).to.equal('email');
       });
 
+      it('should not override custom pattern', () => {
+        expect(fixture('custom-pattern').pattern).to.equal('.+@example\\.com');
+      });
+
       describe('validation', () => {
         describe('valid email addresses', () => {
           validAddresses.map(address => {