Fix uri-decode behavior for "+"

* module/web/uri.scm (uri-decode): Add #:decode-plus-to-space? keyword
  argument.
  (split-and-decode-uri-path): Don't decode plus to space.
* doc/ref/web.texi (URIs): Update documentation.
* test-suite/tests/web-uri.test ("decode"): Add tests.
* NEWS: Add entry.

Based on a patch by Brent <brent@tomski.co.za>.

doc/ref/web.texi

@@ -245,7 +245,7 @@ serialization.
 Declare a default port for the given URI scheme.
 @end deffn
 
-@deffn {Scheme Procedure} uri-decode str [#:encoding=@code{"utf-8"}]
+@deffn {Scheme Procedure} uri-decode str [#:encoding=@code{"utf-8"}] [#:decode-plus-to-space? #t]
 Percent-decode the given @var{str}, according to @var{encoding}, which
 should be the name of a character encoding.
 
@@ -262,6 +262,11 @@ decoded bytes are not valid for the given encoding. Pass @code{#f} for
 @xref{Ports, @code{set-port-encoding!}}, for more information on
 character encodings.
 
+If @var{decode-plus-to-space?} is true, which is the default, also
+replace instances of the plus character @samp{+} with a space character.
+This is needed when parsing @code{application/x-www-form-urlencoded}
+data.
+
 Returns a string of the decoded characters, or a bytevector if
 @var{encoding} was @code{#f}.
 @end deffn