Add hostname_rewrite fields to the Proxy Update endpoint

Current behaviour
The field hostname_rewrite (Host Header) is missing from the 3scale admin portal UI Proxy Update 3scale API endpoint.

Expected behaviour
Find in the 3scale admin portal UI Proxy Update 3scale API endpoint the field hostname_rewrite in the list of available parameters to update the Host Header (since it's a permitted param as defined here).

Workaround
Add in the curl request the parameter hostname_rewrite

curl -v -X PATCH "https://{tenant-name}-admin.{wildcard-domain}/admin/api/services/{service_id}/proxy.xml" -d 'access_token={Access_Token}&hostname_rewrite=example.com'