gzip: use path-add-extension as default

instead of `string-append`-ing the extension,
 because that doesn't work for all path strings

pkgs/racket-doc/file/scribblings/gzip.scrbl

@@ -8,13 +8,17 @@ utilities to create archive files in @exec{gzip} format, or simply to
 compress data using the @exec{pkzip} ``deflate'' method.}
 
 @defproc[(gzip [in-file path-string?]
-               [out-file path-string? (string-append in-file ".gz")])
+               [out-file path-string? (path-add-extension in-file ".gz" #".")])
          void?]{
 
 Compresses data to the same format as the @exec{gzip} utility, writing
 the compressed data directly to a file. The @racket[in-file] argument
 is the name of the file to compress. If the file named by
-@racket[out-file] exists, it will be overwritten.}
+@racket[out-file] exists, it will be overwritten.
+
+@history[#:changed "6.8.0.2"
+         @elem{Changed default expression of @racket[out-file] to use
+               @racket[path-add-extension] instead of @racket[string-append].}]}
 
 
 @defproc[(gzip-through-ports [in input-port?]

racket/collects/file/gzip.rkt

@@ -2208,7 +2208,7 @@
 
 (define gzip
   (case-lambda
-   [(infile) (gzip infile (string-append infile ".gz"))]
+   [(infile) (gzip infile (path-add-extension infile ".gz" #"."))]
    [(infile outfile) ((car (code)) infile outfile)]))
 
 (define (gzip-through-ports in out origname time_stamp)