Fix: Label update operation is executed as insert

When updating an existing label value with command like

 ssh -p 29418 user@review gerrit review <change>,<patchset> --label code-review=+1

where the lower case label name will make the update operation
become insert operation.  This causes an exception:

  "Unique index or primary key violation"

Use standard label name instead of lower case name.

Bug: Issue 2515
Change-Id: I735a5f84d0569554c050b60b8984ee83aa941d39