web: Be less strict when parsing entity tags.

* module/web/http.scm (parse-entity-tag): Be less strict, accepting
  unquoted strings as well.

* test-suite/tests/web-http.test ("response headers"): Add a test for
  etag parsing.

module/web/http.scm

@@ -848,10 +848,15 @@ as an ordered alist."
     (display-digits (date-second date) 2 port)
     (display " GMT" port)))
 
+;; Following https://tools.ietf.org/html/rfc7232#section-2.3, an entity
+;; tag should really be a qstring.  However there are a number of
+;; servers that emit etags as unquoted strings.  Assume that if the
+;; value doesn't start with a quote, it's an unquoted strong etag.
 (define (parse-entity-tag val)
-  (if (string-prefix? "W/" val)
+  (cond
-      (cons (parse-qstring val 2) #f)
+   ((string-prefix? "W/" val) (cons (parse-qstring val 2) #f))
-      (cons (parse-qstring val) #t)))
+   ((string-prefix? "\"" val) (cons (parse-qstring val) #t))
+   (else (cons val #t))))
 
 (define (entity-tag? val)
   (and (pair? val)

test-suite/tests/web-http.test

@@ -353,6 +353,7 @@
   (pass-if-parse age "30" 30)
   (pass-if-parse etag "\"foo\"" '("foo" . #t))
   (pass-if-parse etag "W/\"foo\"" '("foo" . #f))
+  (pass-if-parse etag "foo" '("foo" . #t))
   (pass-if-parse location "http://other-place"
                  (build-uri 'http #:host "other-place"))
   (pass-if-parse proxy-authenticate "Basic realm=\"guile\""