\Apigee\Mint\DeveloperRatePlan::save('update') causes a response exception

Calling method with "update" causes a response exception because API endpoint does not support PUT method.

` ~ http PUT [HOST]/v1/mint/organizations/[ORG]/developers/[DEVELOPER_ID]/developer-rateplans -a [DEVELOPER_ID]`

```
HTTP/1.1 405 Method Not Allowed
Allow: POST,GET,OPTIONS,HEAD
Content-Length: 0
Date: Wed, 03 May 2017 12:54:32 GMT
```