Trimmed vtables of trailing zeroes.

This is something the format supports, but none of the builders
were doing. Can save 10-20% on FlatBuffer binary size!

Also fixed the Go tests.

Change-Id: I616c56ce9bbcfcaee23aa24f0532fcb60b6a8c75
Tested: on Linux.

src/reflection.cpp

@@ -481,7 +481,7 @@ Offset<const Table *> CopyTable(FlatBufferBuilder &fbb,
     fbb.ClearOffsets();
     return fbb.EndStruct();
   } else {
-    return fbb.EndTable(start, static_cast<voffset_t>(fielddefs->size()));
+    return fbb.EndTable(start);
   }
 }