Fix x-www-form-urlencoded with missing values

These produced an exception during parsing.

lib/Cro/HTTP/BodyParsers.pm6

@@ -59,7 +59,7 @@ class Cro::HTTP::BodyParser::WWWFormUrlEncoded does Cro::BodyParser {
                 my $encoding = self.default-encoding;
                 for $payload.split('&') -> Str $string {
                     my Str $string-sp = $string.subst('+', ' ', :g);
-                    my int $eq-idx = $string-sp.index('=');
+                    my int $eq-idx = $string-sp.index('=') // -1;
                     my Str $name = $eq-idx >= 0
                         ?? $string-sp.substr(0, $eq-idx)
                         !! $string-sp;

t/http-request-parser.t

@@ -716,6 +716,19 @@ parses 'A _charset_ in application/x-www-form-urlencoded overrides configured de
             'Values were decoded as utf-8, not latin-1 default, due to _charset_';
     };
 
+parses 'No errors on keys with empty value or missing value',
+        q:to/REQUEST/.chop,
+    POST /bar HTTP/1.1
+    Content-type: application/x-www-form-urlencoded; charset=UTF-8
+    Content-length: 4
+
+    a=&b
+    REQUEST
+    tests => {
+            my $body = .body.result;
+            is-deeply $body.list, (a => '', b => '');
+        };
+
 parses 'Simple multipart/form-data',
     q:to/REQUEST/, :body-crlf,
     POST /bar HTTP/1.1