fix: throw error when Content-Type is invalid

When calling res.set('Content-Type', value) where value is not a valid
MIME type (e.g., 'some-custom-type'), mime.contentType() returns false.
Previously, this false value was coerced to the string "false" and set
as the Content-Type header.

This change throws a TypeError when mime.contentType() returns false,
making the error explicit and easier to catch. This behavior aligns
with the existing TypeError thrown for Array values.

Fixes #7034

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: aviu16

lib/response.js

@@ -673,7 +673,11 @@ res.header = function header(field, val) {
       if (Array.isArray(value)) {
         throw new TypeError('Content-Type cannot be set to an Array');
       }
-      value = mime.contentType(value)
+      var contentType = mime.contentType(value);
+      if (contentType === false) {
+        throw new TypeError('invalid content type');
+      }
+      value = contentType;
     }
 
     this.setHeader(field, value);