objectify.E.foo(CDATA('')) doesn't work

`builder.E(tag, cdata)` was fixed in #1330525, but `objectify.E(tag, cdata)` doesn't work.
Checked in 4.1.0, 3.8.0, and 3.3.6. Compare:

```
In [32]: for module in objectify, builder:
    ...: print tostring(module.E('b', etree.CDATA('hello %s' % module.__name__)))
    ...:

<b xmlns:py="http://codespeak.net/lxml/objectify/pytype" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" py:pytype="str">&lt;lxml.etree.CDATA object at 0x7f71a042d708&gt;</b>
<b><![CDATA[hello lxml.builder]]></b>

In [41]: print("%-20s: %s" % ('Python', sys.version_info))
    ...: print("%-20s: %s" % ('lxml.etree', etree.LXML_VERSION))
    ...: print("%-20s: %s" % ('libxml used', etree.LIBXML_VERSION))
    ...: print("%-20s: %s" % ('libxml compiled', etree.LIBXML_COMPILED_VERSION))
    ...: print("%-20s: %s" % ('libxslt used', etree.LIBXSLT_VERSION))
    ...: print("%-20s: %s" % ('libxslt compiled', etree.LIBXSLT_COMPILED_VERSION))
    ...: ## -----
    ...:
Python : sys.version_info(major=2, minor=7, micro=12, releaselevel='final', serial=0)
lxml.etree : (4, 1, 0, 0)
libxml used : (2, 9, 5)
libxml compiled : (2, 9, 5)
libxslt used : (1, 1, 30)
libxslt compiled : (1, 1, 30)
```