Include boudary=... in mutipart postData

While the HAR spec is not very explicit and their example shows just this one example: ```json
"postData": {
    "mimeType": "multipart/form-data"
}
```
Would it not make sense to include all the information necessary to parse out the post data `text`. Eg.
```json
"postData": {
           "text": "--xYzZY\r\nContent-Disposition: form-data; name=\"sort1\"\r\n\r\noldest date first\r\n--xYzZY--\r\n",
           "mimeType": "multipart/form-data; boundary=xYzZY"
         },
```
Currently, full mimeType is included only in `content-type` request header.

Elsewhere in HAR spec they include the 'extras', eg ```json
"content": {
    "mimeType": "text/html; charset=utf-8"
}
``` 
So one could argue that `mimeType` should include all information necessary to interpret the data. In case of `multipart/form-data`, as per RFC2046 http://www.ietf.org/rfc/rfc2046.txt
```
 The Content-Type field for multipart entities requires one parameter, "boundary".
```
I believe that earlier incarnations, eg `har_exporter.py` included it in the mimeType.

examples/har_dump.py

@@ -140,7 +140,7 @@ def response(flow):
             for a, b in flow.request.urlencoded_form.items(multi=True)
         ]
         entry["request"]["postData"] = {
-            "mimeType": flow.request.headers.get("Content-Type", "").split(";")[0],
+            "mimeType": flow.request.headers.get("Content-Type", ""),
             "text": flow.request.get_text(strict=False),
             "params": params
         }