Fix new version of `create_dir_all`

It will now correctly fail on existing non-directories.

src/libstd/fs.rs

@@ -1777,7 +1777,7 @@ impl DirBuilder {
     fn create_dir_all(&self, path: &Path) -> io::Result<()> {
         match self.inner.mkdir(path) {
             Ok(()) => return Ok(()),
-            Err(ref e) if e.kind() == io::ErrorKind::AlreadyExists => return Ok(()),
+            Err(ref e) if e.kind() == io::ErrorKind::AlreadyExists && path.is_dir() => return Ok(()),
             Err(ref e) if e.kind() == io::ErrorKind::NotFound => {}
             Err(e) => return Err(e),
         }
@@ -1787,7 +1787,7 @@ impl DirBuilder {
         }
         match self.inner.mkdir(path) {
             Ok(()) => Ok(()),
-            Err(ref e) if e.kind() == io::ErrorKind::AlreadyExists => Ok(()),
+            Err(ref e) if e.kind() == io::ErrorKind::AlreadyExists && path.is_dir() => Ok(()),
             Err(e) => Err(e),
         }
     }