Fix serializable trace legacy JSON format

lib/ddtrace/transport/serializable_trace.rb

@@ -31,7 +31,7 @@ def to_msgpack(packer = nil)
       # JSON serializer interface.
       # Used by older version of the transport.
       def to_json(*args)
-        trace.spans.map(&:to_hash).to_json(*args)
+        trace.spans.map { |s| SerializableSpan.new(s).to_hash }.to_json(*args)
       end
     end
 
@@ -102,7 +102,7 @@ def to_msgpack(packer = nil)
       # JSON serializer interface.
       # Used by older version of the transport.
       def to_json(*args)
-        span.to_hash.to_json(*args)
+        to_hash.to_json(*args)
       end
 
       # Used for serialization
@@ -111,6 +111,10 @@ def time_nano(time)
         time.to_i * 1000000000 + time.nsec
       end
 
+      def to_hash
+        span.to_hash
+      end
+
       # Used for serialization
       # @return [Integer] in nanoseconds since Epoch
       def duration_nano(duration)

spec/ddtrace/transport/serializable_trace_spec.rb

@@ -111,12 +111,11 @@
         end
       end
 
-      # DEV: confirm?
       context 'when given 128 bits trace id' do
         let(:trace_id) { 0xaaaaaaaaaaaaaaaaffffffffffffffff }
 
-        it 'serializes 128 bits trace id entirely' do
-          expect(unpacked_trace.map { |s| s['trace_id'] }).to all(eq(0xaaaaaaaaaaaaaaaaffffffffffffffff))
+        it 'serializes the low order 64 bits trace id' do
+          expect(unpacked_trace.map { |s| s['trace_id'] }).to all(eq(0xffffffffffffffff))
         end
       end
     end