[Bug Report] Form validation rule is invalid

### Element UI version
2.13.0

### OS/Browsers version
Win10 / Microsoft Edge 42.17134.1.0

### Vue version
2.6.10

### Reproduction Link
https://codepen.io/zero-cao/pen/VwYByjd

### Steps to reproduce
1. setup VUE/CLI project

2. Go to VUE component section HTML
2.1. Create an el-form with property [:rules="validate"]
2.2. Assign property [prop="commonNname"] the an el-form-item which is validated

3. Go to VUE component section JS
3.1 Add the below code to VUE property [data () {}]
validate: {commonName: [{required: true, message: 'Please input invalid common name', trigger: 'blur'}]}
3.2 something else to do

4. Run the code and test with opening browser

### What is Expected?
1. Input "test" in the form item 'Common Name', the go to other item
2. No error message popup, because this item validation rule is 'required', and it has value 'test'.

### What is actually happening?
1. Input "test" in the form item 'Common Name', the go to other item
2. The red error message 'Please input invalid common name' will popup.