Fix: specs for URI.escape where using unsafe strings as example names

This caused an error in a JUnitFormatter, which was assuming the spec name was a valid string.

spec/std/uri_spec.cr

@@ -121,7 +121,6 @@ describe "URI" do
       {"hello%252%2Bworld", "hello%2+world"},
       {"%E3%81%AA%E3%81%AA", "なな"},
       {"%27Stop%21%27%20said%20Fred", "'Stop!' said Fred"},
-      {"%FF", String.new(1) { |buf| buf.value = 255_u8; {1, 0} }},
       {"%0A", "\n"},
     ].each do |tuple|
       from, to = tuple
@@ -136,6 +135,20 @@ describe "URI" do
       end
     end
 
+    describe "invalid utf8 strings" do
+      input = String.new(1) { |buf| buf.value = 255_u8; {1, 0} }
+
+      it "escapes without failing" do
+        URI.escape(input).should eq("%FF")
+      end
+
+      it "escapes to IO without failing" do
+        String.build do |str|
+          URI.escape(input, str)
+        end.should eq("%FF")
+      end
+    end
+
     it "escape space to plus when space_to_plus flag is true" do
       URI.escape("hello world", space_to_plus: true).should eq("hello+world")
       URI.escape("'Stop!' said Fred", space_to_plus: true).should eq("%27Stop%21%27+said+Fred")