FEAT: allow DEHEX to accept binary! value

So one can use it to properly url-decode utf-8 sequence, like:
```
>> to-string dehex #{2F666F726D3F763D254335253939}
== "/form?v=ř"

>> to-string dehex to-binary "%c5%99"
== "ř"

```

Related issue: metaeducation/rebol-issues#1986

Author: Oldes

src/boot/natives.r

@@ -520,8 +520,8 @@ uppercase: native [
 ]
 
 dehex: native [
-	{Converts URL-style hex encoded (%xx) strings.}
-	value [any-string!] {The string to dehex}
+	{Converts URL-style hex encoded (%xx) strings. If input is UTF-8 encode, you should first convert it to binary!}
+	value [any-string! binary!] {The string to dehex}
 ]
 
 get: native [

src/tests/units/series-test.r3

@@ -218,6 +218,15 @@ Rebol [
 
 ===end-group===
 
+
+===start-group=== "DEHEX"
+
+--test-- "DEHEX UTF-8 encoded data"
+	;@@ https://github.com/rebol/rebol-issues/issues/1986
+	--assert "řek" = to-string dehex to-binary "%c5%99ek"
+
+===end-group===
+
 ;-- VECTOR related tests moved to %vector-test.r3
 
 ~~~end-file~~~