fix(res): preserve original value when mime.contentType() returns false

[speedcoder430]

fix: prevent res.set('Content-Type') from setting header to literal string 'false'

When calling res.set('Content-Type', value) with a value that mime.contentType() cannot resolve (e.g., 'some-custom-type', it returns false. This false was passed directly to setHeader(), which coerced it to the string "false" — resulting in Content-Type: false in the response headers.

This change falls back to the original value when mime.contentType() returns false, consistent with how res.type() already handles this case.

Changes:

• response.js : mime.contentType(value) → mime.contentType(value) || value
• res.set.js : Added test to verify the original value is preserved for unrecognized types

Fixes #7034

[krzysdz]

Duplicate of #7035