If the vMerge does not have a 'val' attibute setting table attributes fails

```
File "/home/jason/third_party_dev/pydocx/pydocx/DocxParser.py" in __init__
  193.         self.parse_begin(self.root)  # begin to parse
File "/home/jason/third_party_dev/pydocx/pydocx/DocxParser.py" in parse_begin
  340.         self._set_table_attributes(el)
File "/home/jason/third_party_dev/pydocx/pydocx/DocxParser.py" in _set_table_attributes
  223.                             'continue' == v_merge.attrib['val']

Exception Type: KeyError 
Exception Value: 'val'
```