Fixes matching error #2418

Fixes the split iterator, the main problem was with the incrementation
of 'last'. Last was first incremented to the index of the first
character after the match, but was then incremented again at the
beginning of the while loop. This caused a problem if that character
after the first match, also matched the regular expression.

lib/impure/re.nim

@@ -373,23 +373,26 @@ iterator split*(s: string, sep: Regex): string =
   ## Results in:
   ##
   ## .. code-block:: nim
+  ##   ""
   ##   "this"
   ##   "is"
   ##   "an"
   ##   "example"
+  ##   ""
   ##
   var
-    first = 0
-    last = 0
+    first = -1
+    last = -1
   while last < len(s):
     var x = matchLen(s, sep, last)
     if x > 0: inc(last, x)
     first = last
+    if x == 0: inc(last)
     while last < len(s):
-      inc(last)
       x = matchLen(s, sep, last)
-      if x > 0: break
-    if first < last:
+      if x >= 0: break
+      inc(last)
+    if first <= last:
       yield substr(s, first, last-1)
 
 proc split*(s: string, sep: Regex): seq[string] =