Can't add headers with same names but with different values for HTTP agent item

Steps to reproduce:

1. Create HTTP agent item
2. Fill required fields
3. Add two headers
   a => 1
   a => 2
4. Save item
5. Edit item

Result:
one header: a => 2
Expected:
two headers:
a => 1
a => 2

The headers for the "Web scenario" in step work as expected.