FIX: do not expecting a Rebol header when input is a string value

resolves: Oldes/Rebol-issues#2596

Author: Oldes

src/mezz/sys-base.reb

@@ -46,11 +46,15 @@ do*: func [
 	;       Currently, load of URL has no special block forms.
 
 	; Load the data, first so it will error before change-dir
-	data: load/header/as value 'unbound ; unbound so DO-NEEDS runs before INTERN
-	; Get the header and advance 'data to the code position
-	hdr: first+ data  ; object or none
-	; data is a block! here, with the header object in the first position back
-	mod?: 'module = select hdr 'type
+	either string? value [
+		data: load/all/as value 'unbound ; does not evaluate REBOL header
+	][
+		data: load/header/as value 'unbound ; unbound so DO-NEEDS runs before INTERN
+		; Get the header and advance 'data to the code position
+		hdr: first+ data  ; object or none
+		; data is a block! here, with the header object in the first position back
+		mod?: 'module = select hdr 'type
+	]
 
 	either all [string? value  not mod?] [
 		; Return result without script overhead

src/tests/units/evaluation-test.r3

@@ -90,6 +90,11 @@ Rebol [
 	;@@ https://github.com/Oldes/Rebol-issues/issues/903
 		--assert all [error? e: try [do "<> 0"]  e/id = 'missing-arg]
 		--assert all [error? e: try [do next [1 <> 0]]  e/id = 'missing-arg]
+
+	--test-- "do string with Rebol header"
+	;@@ https://github.com/Oldes/Rebol-issues/issues/2596
+		--assert "Rebol []" == do "{Rebol []}"
+		--assert "^/Rebol []" == try [do "{^/Rebol []}"]
 
 ===end-group===