White space at the beginning of a file should be encoded in filesystem store

https://msdn.microsoft.com/en-us/library/windows/desktop/aa365247.aspx#naming_conventions only mention white space at the end of a file but Windows Explorer does not allow setting a white space at the beginning of a file either.

Note that the filesystem supports it so it's not causing any issue at XWiki level but it's still better to produce something the Windows Explorer can manipulate just in case.