Fix median sort, set-op duplicates, and unescape Unicode range

Fixes three independent bugs:

- Arithmetic.mjs (median): sort was only applied for even-length arrays;
  odd-length inputs returned the middle element of the unsorted array.
  Sort is now unconditional. Fixes #2239.

- SetDifference / SetIntersection: plain .filter() on array a preserved
  duplicate entries from the first input, violating set semantics.
  Deduplicate a via [...new Set(a)] before filtering. Fixes #2241.

- UnescapeUnicodeCharacters: regex quantifier was hardcoded to {4} for
  all prefixes. U+ notation allows 4-6 hex digits (e.g. U+1F600 for 😀);
  \u and %u remain at exactly {4} per their respective specs. Fixes #2242.

src/core/lib/Arithmetic.mjs

@@ -108,10 +108,11 @@ export function mean(data) {
  * @returns {BigNumber}
  */
 export function median(data) {
-    if ((data.length % 2) === 0 && data.length > 0) {
+    if (data.length === 0) return data[0];
     data.sort(function(a, b) {
         return a.minus(b);
     });
+    if ((data.length % 2) === 0) {
         const first = data[Math.floor(data.length / 2)];
         const second = data[Math.floor(data.length / 2) - 1];
         return mean([first, second]);

src/core/operations/SetDifference.mjs

@@ -75,7 +75,7 @@ class SetDifference extends Operation {
      * @returns {Object[]}
      */
     runSetDifference(a, b) {
-        return a
+        return [...new Set(a)]
             .filter((item) => {
                 return b.indexOf(item) === -1;
             })

src/core/operations/SetIntersection.mjs

@@ -75,7 +75,7 @@ class SetIntersection extends Operation {
      * @returns {Object[]}
      */
     runIntersect(a, b) {
-        return a
+        return [...new Set(a)]
             .filter((item) => {
                 return b.indexOf(item) > -1;
             })

src/core/operations/UnescapeUnicodeCharacters.mjs

@@ -56,7 +56,8 @@ class UnescapeUnicodeCharacters extends Operation {
      */
     run(input, args) {
         const prefix = prefixToRegex[args[0]],
-            regex = new RegExp(prefix+"([a-f\\d]{4})", "ig");
+            quantifier = args[0] === "U+" ? "{4,6}" : "{4}",
+            regex = new RegExp(prefix+"([a-f\\d]"+quantifier+")", "ig");
         let output = "",
             m,
             i = 0;