Added missing cursor increment in rfc1867PostHandler. Fixes #1706

Added missing cursor increment in rfc1867PostHandler. Fixes #1706

Summary:
The missing multipart_buffer cursor increment causes some of the post data to be duplicated when read from the buffer.

Here is one example of the resulting data in the buffer
```
------WebKitFormBoundary8pfe3ho8C5p2ZXwA
Content-Disposition: form-data; name="single-0"

0
------WebKitFormBoundary8pfe3ho8C5p2ZXwA
Content-Disposition: form-data; name="single-1"

1
------WebKitFormBoundary8pfe3ho8C5p2ZXwA
Content-Disposition: form-data; name="single-2"

2
------WebKitFormBoundary8pfe3ho8C5p2ZXwA
Content-Disposition: form-data; name="single-3"

3
------WebKitFormBoundary8pfe3ho8C5p2ZXwA
Content-Disposition: form-data; name="single-4"

4
------WebKitFormBoundary8pfe3ho8C5p2ZXwA
Content-Disposition: form-data; name="single-5"

5
------WebKitFormBoundary8pfe3ho8C5p2ZXwA
Content-Disposition: form-data; name="single-6"

6
------WebKitFormBoundary8pfe3ho8C5p2ZXwA
Content-Disposition: form-data; name="single-7"

7
------WebKitFormBoundary8pfe3ho8C5p2ZXwA--

4"

4
------WebKitFormB
Closes https://github.com/facebook/hhvm/pull/6585

Reviewed By: paulbiss

Differential Revision: D2695893

fb-gh-sync-id: 60d545133f03439f8f754a00c0f5454736619747