rename invalid property name

"svndumpfilter generated" is an invalid property name for Subversion even if it's not clear stated in the svnbook http://svnbook.red-bean.com/en/1.7/svn.advanced.props.html. There is already a bug report for this issue in svnbook: https://code.google.com/p/svnbook/issues/detail?id=157

The space makes problems in different places:
* CLI: 'svndumpfilter generated' is not a valid Subversion property name
```
svn pg 'svndumpfilter generated' .
svn: 'svndumpfilter generated' is not a valid Subversion property name
```
* mod_dav_svn: generates invalid XML
```
<C:svndumpfilter generated>True</C:svndumpfilter generated>
```