plugin/file: allow README.md testing (#3052)

* Fix corefile usage

* plugin/file: allow README.md testing

Allow readme testing for the file plugin and fix bugs that where found:

* the reader wasn't reset when re-reading the same io.reader for a
  different origin.

Signed-off-by: Miek Gieben <miek@miek.nl>

* Update test/example_test.go

Co-Authored-By: Michael Grosser <development@stp-ip.net>

plugin/file/README.md

@@ -44,7 +44,7 @@ file DBFILE [ZONES... ] {
 Load the `example.org` zone from `example.org.signed` and allow transfers to the internet, but send
 notifies to 10.240.1.1
 
-~~~ txt
+~~~ corefile
 example.org {
     file example.org.signed {
         transfer to *
@@ -55,7 +55,7 @@ example.org {
 
 Or use a single zone file for multiple zones:
 
-~~~ txt
+~~~ corefile
 . {
     file example.org.signed example.org example.net {
         transfer to *
@@ -67,7 +67,7 @@ Or use a single zone file for multiple zones:
 Note that if you have a configuration like the following you may run into a problem of the origin
 not being correctly recognized:
 
-~~~ txt
+~~~ corefile
 . {
     file db.example.org
 }
@@ -78,7 +78,7 @@ which, in this case, is the root zone. Any contents of `db.example.org` will the
 origin set; this may or may not do what you want.
 It's better to be explicit here and specify the correct origin. This can be done in two ways:
 
-~~~ txt
+~~~ corefile
 . {
     file db.example.org example.org
 }
@@ -86,7 +86,7 @@ It's better to be explicit here and specify the correct origin. This can be done
 
 Or
 
-~~~ txt
+~~~ corefile
 example.org {
     file db.example.org
 }