Incoming mail with priority flag always sets ticket priority using Default Priority Scheme instead of custom priority scheme

Issue Summary  

Incoming mail with priority flag always sets ticket priority using Default Priority Scheme instead of custom priority scheme.

Steps to Reproduce

1. Create new priority scheme using eg. 3 different priorities and associate it with a project
   
2. Set up incoming mail handler and associate it with the project
3. Send 3 emails: one with low, other with high and the thrid one without priority set
4. Issues get created

Expected Results

Each mail creates a ticket and assign a calculated priority according to the associated priority scheme.

Actual Results

Only mail without priority flag creates ticket with the default priority set from project's priority scheme.
Other two mails create issues and assign priorites which are non-existent in associated priority scheme, but they are defined in Default Priority Scheme.

Example the ticket created from the email sent with High priority flag, the priority has been marked as "(not available)":

Workaround

Currently there is no known workaround for this behavior. A workaround will be added here when available