Allow IPv6 proxy hosts starting with [.

Altered the regular expression that checked for a sane-ish looking
URL for our proxy host:

 if $url =~ /^https?:/ && $url !~ m,^https?://\w,;

Given this expression, a proxy host in the form of
http://[::1]:33485/ would fail because it starts with a left bracket.

The expression above has been altered to:

 if $url =~ /^https?:/ && $url !~ m,^https?://[\w[],;

Which will allow it to begin with a word character or a left bracket.

This will resolve issue #237

Changes

@@ -2,6 +2,7 @@ Change history for libwww-perl
 
 {{$NEXT}}
     - Improve ->add_handler documentation (GH #272) (Julien Fiegehenn)
+    - Alter our rule set to allow IPv6 proxy hosts beginning with [. (GH #237)
 
 6.36      2018-10-10 02:20:58Z
     - fix broken link https://metacpan.org/pod/LWP::Simple by fixing pod

lib/LWP/UserAgent.pm

@@ -1059,7 +1059,7 @@ sub proxy {
         my $url = shift;
         if (defined($url) && length($url)) {
             Carp::croak("Proxy must be specified as absolute URI; '$url' is not") unless $url =~ /^$URI::scheme_re:/;
-            Carp::croak("Bad http proxy specification '$url'") if $url =~ /^https?:/ && $url !~ m,^https?://\w,;
+            Carp::croak("Bad http proxy specification '$url'") if $url =~ /^https?:/ && $url !~ m,^https?://[\w[],;
         }
         $self->{proxy}{$key} = $url;
         $self->set_my_handler("request_preprepare", \&_need_proxy)