Some small optimisations to textutils.urlEncode

This probably isn't useful in practice — nobody is escaping 1MB of data.
Right. Right???? But no harm in doing it.

 - Cache globals as locals.
 - Remove redundant pattern capture.
 - Merge string.format calls into one.

Also remove the "if str then" check. I assume we accepted nil values a
long time ago, but that was broken when we added arg checks. Woops!

projects/core/src/main/resources/data/computercraft/lua/rom/apis/textutils.lua

@@ -944,22 +944,21 @@ unserialiseJSON = unserialise_json
 -- @since 1.31
 function urlEncode(str)
     expect(1, str, "string")
-    if str then
-        str = string.gsub(str, "\n", "\r\n")
-        str = string.gsub(str, "([^A-Za-z0-9 %-%_%.])", function(c)
-            local n = string.byte(c)
-            if n < 128 then
-                -- ASCII
-                return string.format("%%%02X", n)
-            else
-                -- Non-ASCII (encode as UTF-8)
-                return
-                    string.format("%%%02X", 192 + bit32.band(bit32.arshift(n, 6), 31)) ..
-                    string.format("%%%02X", 128 + bit32.band(n, 63))
-            end
-        end)
-        str = string.gsub(str, " ", "+")
-    end
+    local gsub, byte, format, band, arshift = string.gsub, string.byte, string.format, bit32.band, bit32.arshift
+
+    str = gsub(str, "\n", "\r\n")
+    str = gsub(str, "[^A-Za-z0-9%-%_%.]", function(c)
+        if c == " " then return "+" end
+
+        local n = byte(c)
+        if n < 128 then
+            -- ASCII
+            return format("%%%02X", n)
+        else
+            -- Non-ASCII (encode as UTF-8)
+            return format("%%%02X%%%02X", 192 + band(arshift(n, 6), 31), 128 + band(n, 63))
+        end
+    end)
     return str
 end
 

projects/core/src/test/resources/test-rom/spec/apis/textutils_spec.lua

@@ -296,6 +296,22 @@ describe("The textutils library", function()
             textutils.urlEncode("")
             expect.error(textutils.urlEncode, nil):eq("bad argument #1 (string expected, got nil)")
         end)
+
+        it("encodes newlines", function()
+            expect(textutils.urlEncode("a\nb")):eq("a%0D%0Ab")
+        end)
+
+        it("leaves normal characters as-is", function()
+            expect(textutils.urlEncode("abcABC0123")):eq("abcABC0123")
+        end)
+
+        it("escapes spaces", function()
+            expect(textutils.urlEncode("a b c")):eq("a+b+c")
+        end)
+
+        it("escapes special characters", function()
+            expect(textutils.urlEncode("a%b\0\255")):eq("a%25b%00%C3%BF")
+        end)
     end)
 
     describe("textutils.complete", function()