Multi-User Picker inserts extra spaces every time a user is selected

The multi-user picker inserts extra spaces between existing usernames every time a new user is added.

Repro steps:
1) Add a multi-user custom field to a project.
2) Create a new issue.
3) Start typing a name. Press <enter> to accept the autocomplete.
4) Repeat step 3 with a different name.
5) Repeat step 3 with a different name. At this point, an extra space will be inserted between the first two names.
6) Repeat step 3 with a different name. Extra spaces will be inserted between the first three names.

For example,

Step 3:     "pwyatt, "
Step 4:     "pwyatt, qazwsx, "
Step 5:     "pwyatt,  qazwsx, admin06, "
Step 6:     "pwyatt,   qazwsx,  admin06, test1111111, "
Eventually: "pwyatt,        qazwsx,       admin06,      test1111111,     testeneto,    afra,   1test,  abcde, testing2009, "