Issue title: input-item value shoule be tostring & defaultValue should not override the value
 Issue body: 

## Version
0.11.6
## Environment
win10&Google Chrome 810-512-3476
## Reproduction link
[https://stackblitz.com/edit/angular-dmcigx?file=src/app/app.component.ts](https://stackblitz.com/edit/angular-dmcigx?file=src/app/app.component.ts)
## Steps to reproduce
1.value = 789,defaultvalue not input
2.Click the delete or number
3.defaultValue should not override the value
## What is expected?
1.2:The value is normally deleted or changed
3: input-item show value 789
## What is actually happening?
1.2:Delete times error, change when the implementation of the plus operation
3:input-item show value '' ,Because the input value is overwritten by the default value
## Other?
![inputitem](https://user-images.githubusercontent.com/34148806/53928548-8542e700-40c5-11e9-899e-a49eac13ecd3.gif)
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 Comments: 
 Comment 0: ![image](https://user-images.githubusercontent.com/34148806/53928855-993b1880-40c6-11e9-96a8-d39c1d734818.png)

 Comment 1: The default might be this:
![image](https://user-images.githubusercontent.com/34148806/53928875-b66fe700-40c6-11e9-86f6-4829b80c49c4.png)

 Comment 2: #330 
 Comment 3: @Anberm has #330  solved your issue?
 Comment 4: @fisherspy @sWhite01111 All right, thank you
{'source': 'GithubIssues', 'repo_name_with_owner': 'NG-ZORRO/ng-zorro-antd-mobile', 'issue_id': 418102898, 'repo_stars': 781.0}