Can no longer override default project-level searches

I found this when trying to show Hossein how it worked. It seems it was introduced in 1.7.5, as I've verified that the bug is present in 1.7.5.1 and 1.7.6-SNAPSHOT, but it works properly in 1.7.4.1. To recreate:

1. Find a project with 1 or more MR Sessions.
2. In the project's data listings, load the default "MR Sessions" tab if not already displayed.
3. Click the "Age" column, then click "Hide Column"
4. Click Options > Save Search, leave the values alone, and save the search.
5. Refresh the page.

In 1.7.4.1, this would result in the project having still 1 searched called "MR Sessions", but with the modified columns. In recent versions of XNAT, it just creates a second search with the same name.