Unable to invite customers with apostophe in email (email not considered valid)

Steps to reproduce

1. Click on the Service Desk project > People > Customers > Invite Customers
2. Enter an email address with apostrophe (john'doe@example.com)
3. Click enter

Expected Results

• Email is considered valid and able to continue typing next email address or send invite

Actual Results

• Error message saying john'doe@example.com is not a valid email address
  
• Unable to invite customers

Notes

• Tested with JIRA 6.3.11 and Service Desk 2.1.1

Current workaround

1. Create the user in JIRA with this email address
   Cog icon > User Management > Users > Create User
2. Remove the user from jira-users group so that he/she won't count towards the license
3. Add this user to the Service Desk Customers Role
   Projects > ServiceDeskProject > Administration > Roles