audit: don't try to HTTP check non-HTTP content.

Library/Homebrew/dev-cmd/audit.rb

@@ -175,6 +175,8 @@ class FormulaAuditor
   end
 
   def self.check_http_content(url, user_agents: [:default])
+    return unless url.start_with? "http"
+
     details = nil
     user_agent = nil
     user_agents.each do |ua|

Library/Homebrew/test/audit_test.rb

@@ -419,9 +419,8 @@ class FormulaAuditorTests < Homebrew::TestCase
     EOS
 
     fa.audit_homepage
-    assert_equal ["The homepage should start with http or https " \
-      "(URL is #{fa.formula.homepage}).", "The homepage #{fa.formula.homepage} is not reachable " \
-      "(HTTP status code 000)"], fa.problems
+    assert_equal ["The homepage should start with http or https (URL is #{fa.formula.homepage})."],
+      fa.problems
 
     formula_homepages = {
       "bar" => "http://www.freedesktop.org/wiki/bar",