Response Headers when Adding should replace a header if it exists already

[GeekInTheNorth]

Affects In-Proc solutions

How to Reproduce:

• Go to Response Headers
• Add a new header for "Cache-Control" with a value of "public"
• Visit a content page

Expected Result:

Cache-Control header should exist once

Actual Result:

Cache-Control header exists twice, once as the header defined by the CMS Administrator and the default no-cache value for content pages.