Transform tree data to `bytes`.

The data in the tree test cannot be automatically converted to `bytes`
as a encoding is necessary in that case:

```python
Traceback (most recent call last):
  File "/srv/slapgrid/slappart66/git/pygolang/golang/__init__.py", line 125, in _
    return f(*argv, **kw)
  File "/srv/slapgrid/slappart66/git/wendelin.core/wcfs/internal/xbtree/xbtreetest/treegen.py", line 281, in TreesSrv
    zblk.setblkdata(v2)
  File "/srv/slapgrid/slappart66/git/wendelin.core/bigfile/file_zodb.py", line 294, in setblkdata
    blkdata = bytes(buf)                    # FIXME does memcpy
TypeError: string argument without an encoding
```

As we know that the data is ASCII, we use the ASCII encoder to convert
it to `bytes`.

wcfs/internal/xbtree/xbtreetest/treegen.py

@@ -260,7 +260,7 @@ def TreesSrv(zstor, r):
             t, D = treetxt.split()
             assert D.startswith('D')
             kv = kvDecode(t[1:], zctx.vdecode)
-            zv = _kvDecode(D[1:], kdecode=lambda ktxt: ktxt, vdecode=lambda vtxt: vtxt)
+            zv = _kvDecode(D[1:], kdecode=lambda ktxt: ktxt, vdecode=lambda vtxt: vtxt.encode('ascii'))
             patch(ztree,    diff(ztree, kv), kv)
 
             # ~ patch(valdict, diff(valdict,zv))  but sets zblk.value on change