go/zodb/fs1/index: Don't rely on []byte being pickled as string

As https://github.com/kisielk/og-rek/pull/57 maybe shows []byte was
pickling as string only unintentionally and that might change.

We are already explicitly checking for string in corresponding index
load place:

	https://lab.nexedi.com/kirr/neo/blob/2dba8607/go/zodb/storage/fs1/index.go#L282

so it is better we also explicitly save the bits as string.

If we don't and https://github.com/kisielk/og-rek/pull/57 gets accepted,
tests will fail:

	--- FAIL: TestIndexSaveLoad (0.00s)
	    index_test.go:176: index load: /tmp/t-index893650059/458967662/1.fs.index: pickle @6: invalid oidPrefix: type []uint8
	Traceback (most recent call last):
	  File "./py/indexcmp", line 41, in <module>
	    main()
	  File "./py/indexcmp", line 29, in main
	    d2 = fsIndex.load(path2)
	  File "/home/kirr/src/wendelin/z/ZODB/src/ZODB/fsIndex.py", line 138, in load
	    data[ensure_bytes(k)] = fsBucket().fromString(ensure_bytes(v))
	  File "/home/kirr/src/wendelin/z/ZODB/src/ZODB/fsIndex.py", line 71, in ensure_bytes
	    return s.encode('ascii') if not isinstance(s, bytes) else s
	AttributeError: 'bytearray' object has no attribute 'encode'
	--- FAIL: TestIndexSaveToPy (0.04s)
	    index_test.go:218: zodb/py read/compare index: exit status 1

go/zodb/storage/fs1/index.go

@@ -133,8 +133,8 @@ func (fsi *Index) Save(w io.Writer) (err error) {
 			if oidPrefix != oidPrefixCur || errStop != nil {
 				// emit (oid[0:6], oid[6:8]oid[6:8]...pos[2:8]pos[2:8]...)
 				binary.BigEndian.PutUint64(oidb[:], uint64(oidPrefixCur))
-				t[0] = oidb[0:6]
-				t[1] = bytes.Join([][]byte{oidBuf, posBuf}, nil)
+				t[0] = string(oidb[0:6])
+				t[1] = string(bytes.Join([][]byte{oidBuf, posBuf}, nil))
 				err = p.Encode(pickle.Tuple(t[:]))
 				if err != nil {
 					return err