V3 invite user requires Content-Type header override

[Jeremy Young]

Using this API that was shared by our account team I've found that the pysnyk client is passing Content-Type: application/json where Content-Type: application/vnd.api+json is expected. Adding an optional parameter to the put and post methods of the low-level client for v3 users to optionally override headers as needed may help solve this.

client.api_post_headers["Content-Type"] = "application/vnd.api+json"