some little improvements

1.fix a problem on **ACWebDAVItem's**  **createDateStr** and **modifyDateStr**,
   it will not return nil now(reference from [DAVKit](https://github.com/mattrajca/DAVKit))

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2.fix a bug on transfering the parameter **overwrite** in **ACWebDAVCopyRequest**;
   It didn't  transfer  **overwrite** successfully  before. 

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3  fix the delegate call of **ACWebDAVCopyRequest** and **ACWebDAVMoveRequest**;

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4.**ACWebDAVDownloadRequest.h&.m**
*  add a new var to hold the reference of **currentConnection**, used to cancel a 
   request if needed;
*  also add a new method ```cancel ``` to cancel this request

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5.**ACWebDAVDeleteRequest.m** 
```  - connection: didReceiveResponse:```
*    response.statusCode=201 OR response.statusCode=204 all shows deleted successfully.

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6.**ACWebDAVClient.h**

* - add new optional delegate methods

```- (BOOL)needToCancelDownload```

```- (void)resetCancelDownload:(BOOL)cancel```

```- (BOOL)needToCancelUpload```

```- (void)resetCancelUpload:(BOOL)cancel```

used to check if need to cancel a request