fix: avoid using initial step in validation

setting the min or max property should not trigger step-based validation

fix #420

src/vaadin-number-field.html

@@ -303,7 +303,13 @@
         }
 
         _stepChanged(step) {
-          this.inputElement.step = step;
+          if (!this.__stepChangedCalled) {
+            // Setting the initial value, avoid using it in validation.
+            this.inputElement.step = 'any';
+            this.__stepChangedCalled = true;
+          } else {
+            this.inputElement.step = step;
+          }
         }
 
         _minChanged(min) {

test/number-field.html

@@ -749,6 +749,42 @@
           expect(numberField.validate(), 'value should not be greater than max').to.be.false;
         });
 
+        it('should validate by step when defined by user', () => {
+          numberField.step = 1.5;
+
+          [-6, -1.5, 0, 1.5, 4.5].forEach(validValue => {
+            numberField.value = validValue;
+            expect(numberField.validate()).to.be.true;
+          });
+
+          [-3.5, -1, 2, 2.5].forEach(invalidValue => {
+            numberField.value = invalidValue;
+            expect(numberField.validate()).to.be.false;
+          });
+        });
+
+        it('should use min as step basis in validation when both are defined', () => {
+          numberField.min = 1;
+          numberField.step = 1.5;
+
+          [1, 2.5, 4, 5.5].forEach(validValue => {
+            numberField.value = validValue;
+            expect(numberField.validate()).to.be.true;
+          });
+
+          [1.5, 3, 5].forEach(invalidValue => {
+            numberField.value = invalidValue;
+            expect(numberField.validate()).to.be.false;
+          });
+        });
+
+        it('should not validate by step when only min and max are set', () => {
+          numberField.min = 1;
+          numberField.max = 5;
+          numberField.value = 1.5; // would be invalid by default step=1
+          expect(numberField.validate()).to.be.true;
+        });
+
         describe('removing validation constraints', () => {
           it('should update "invalid" state when "min" is removed', () => {
             numberField.value = '42';