fix: res.set('Content-Type') silently sets header to literal string

[JiwaniZakir]

Fixes #7034

When res.set('Content-Type', value) was called with an unrecognized MIME type, mime.contentType() returned false, and that false value was passed directly to this.setHeader(), resulting in the literal string 'false' being set as the Content-Type header. The fix in lib/response.js changes the assignment in res.header to mime.contentType(value) || value, falling back to the original value when mime.contentType() returns a falsy result. This preserves the existing behavior for recognized types while allowing unrecognized but valid Content-Type strings (e.g. custom or vendor types) to pass through unchanged. Verified against the reported reproduction case where res.set('Content-Type', 'application/vnd.custom') previously set the header to 'false'.

[krzysdz]

Duplicate of #7035

[krzysdz]

Verified against the reported reproduction case where res.set('Content-Type', 'application/vnd.custom') previously set the header to 'false'.

This works even without this change and the issue (and my comment) explicitly mention that the problem occurs when the "type" does not contain a / character.