addAttachment does not link the uploaded attachment to the workitem

rtcclient 0.7.0
RTC 6.0.6.1

When adding an attachment, the file is successfully uploaded, but when linking to the workitem the server complains about malformed XML.

To get it to work I fixed `_add_attachment_link` to use JSON:
```python
def _add_attachment_link(self, attachment_info):
headers = copy.deepcopy(self.rtc_obj.headers)
headers["Content-Type"] = self.OSLC_CR_JSON

payload = {"rdf:resource": attachment_info["url"],
"dcterms:title": ": ".join([str(attachment_info["id"]),
attachment_info["name"]])}
attachment_tag = ("/rtc_cm:com.ibm.team.workitem.linktype."
"attachment.attachment")
attachment_collection_url = self.url + attachment_tag

resp = self.post(attachment_collection_url,
json.dumps(payload),
verify=False,
headers=headers,
proxies=self.rtc_obj.proxies)
raw_data = xmltodict.parse(resp.content)

return Attachment(attachment_info["url"],
self.rtc_obj,
raw_data=raw_data["rtc_cm:Attachment"])
```