fix: throw error when Content-Type is invalid

when you call res.set('Content-Type', value) with something thats not a
real MIME type, mime.contentType() returns false and that false was getting
coerced to the string "false" and set as the header. now it throws a
TypeError instead.

fixes #7034

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);