Changing code default value changes default value for all codes

Changing a code's default value changes the default value for all codes to that default.

To reproduce:
1. Create two codes, say code1 and code2.
2. Add a single option "x" to code1 and make it the default (default_single).
3. Add a single option "y" to code2 and make it the default (default_single).
4. Go back to the code1 options.

Expected result: "x" is still the default value for code1.
Actual result: code1 has no default value.

Behind the scenes: code1 default value has also been changed in the database to "y".

A patch is attached.