Invalid golang code generated for optional set<binary> with a default value

There's a bug in the Thrift Go generator that happens when an optional set<bynary> is used with a default value. For example, a simple struct like this:

struct OptionalSetBinary {
  1: optional set<binary> bin = {}
}

...generates a default value like this:

var OptionalSetBinary_Bin_DEFAULT [][]byte = []string{
}

...and fails the compilation of the unit-test:

../src/gen/thrifttest/ThriftTest.go:5620:5: cannot use []string{} (type []string) as type [][]byte in assignment