avoid adding repeated cf values

now RT::ObjectCustomFieldValues::HasEntry compares LargeContent and also
canonicalizes values before comparision when possible.

see also #4553

btw, there was a bug that if you change a date cf from "2014-05-15" to
"today"(which also means "2014-05-15" at that time), you will get a confusing
message saying "2014-05-15 changed to 2014-05-15". this commit fixes it too.
tests for repeated values
HasEntry canonicalize values now, so no need to explicitly do that any more
Merge branch '4.0/cf-repeated-values' into 4.0-trunk