A resource which does not define content_types_provided() will blow up on request

[autarch]

Here's an example stack trace:

Can't use an undefined value as an ARRAY reference at /home/autarch/.perlbrew/libs/perl-5.22.0@dev/lib/perl5/HTTP/Headers/ActionPack/Core/BaseHeaderType.pm line 38.
    HTTP::Headers::ActionPack::Core::BaseHeaderType::new_from_string("HTTP::Headers::ActionPack::MediaType", undef) called at /home/autarch/.perlbrew/libs/perl-5.22.0@dev/lib/perl5/HTTP/Headers/ActionPack.pm line 87
    HTTP::Headers::ActionPack::create(HTTP::Headers::ActionPack=HASH(0x14c13e0), "MediaType") called at /home/autarch/projects/webmachine-perl/lib/Web/Machine/Util.pm line 32
    Web::Machine::Util::create_header("MediaType") called at /home/autarch/projects/webmachine-perl/lib/Web/Machine/FSM/States.pm line 235
    Web::Machine::FSM::States::c3(My::Resource::Test701=HASH(0x16d9428), Plack::Request=HASH(0x111e860), Plack::Response=HASH(0x161d358), HASH(0x18b1040)) called at /home/autarch/projects/webmachine-perl/lib/Web/Machine/FSM.pm line 62
    Web::Machine::FSM::try {...} () called at /home/autarch/.perlbrew/libs/perl-5.22.0@dev/lib/perl5/Try/Tiny.pm line 81
    eval {...} called at /home/autarch/.perlbrew/libs/perl-5.22.0@dev/lib/perl5/Try/Tiny.pm line 72
    Try::Tiny::try(CODE(0x18b1058), Try::Tiny::Catch=REF(0x16e9f60)) called at /home/autarch/projects/webmachine-perl/lib/Web/Machine/FSM.pm line 129
    Web::Machine::FSM::run(Web::Machine::FSM=HASH(0x1857750), My::Resource::Test701=HASH(0x16d9428)) called at /home/autarch/projects/webmachine-perl/lib/Web/Machine.pm line 93
    Web::Machine::call(Web::Machine=HASH(0xbe26b0), HASH(0x161d400)) called at /home/autarch/.perlbrew/libs/perl-5.22.0@dev/lib/perl5/Plack/Component.pm line 50
    Plack::Component::__ANON__[/home/autarch/.perlbrew/libs/perl-5.22.0@dev/lib/perl5/Plack/Component.pm:50](HASH(0x161d400)) called at /home/autarch/.perlbrew/libs/perl-5.22.0@dev/lib/perl5/Plack/Test/MockHTTP.pm line 24
    Plack::Test::MockHTTP::try {...} () called at /home/autarch/.perlbrew/libs/perl-5.22.0@dev/lib/perl5/Try/Tiny.pm line 79
    eval {...} called at /home/autarch/.perlbrew/libs/perl-5.22.0@dev/lib/perl5/Try/Tiny.pm line 72
    Try::Tiny::try(CODE(0x1695738), Try::Tiny::Catch=REF(0x1122f50)) called at /home/autarch/.perlbrew/libs/perl-5.22.0@dev/lib/perl5/Plack/Test/MockHTTP.pm line 27
    Plack::Test::MockHTTP::request(Plack::Test::MockHTTP=HASH(0x180c218), HTTP::Request=HASH(0x180c188)) called at /home/autarch/.perlbrew/libs/perl-5.22.0@dev/lib/perl5/Plack/Test.pm line 38
    Plack::Test::__ANON__(HTTP::Request=HASH(0x180c188)) called at t/701-content-type-is-actionpack.t line 52
    main::__ANON__[t/701-content-type-is-actionpack.t:53](CODE(0x180c230)) called at /home/autarch/.perlbrew/libs/perl-5.22.0@dev/lib/perl5/Plack/Test.pm line 38
    Plack::Test::test_psgi("app", CODE(0xbe2758), "client", CODE(0x180bb70)) called at t/701-content-type-is-actionpack.t line 54

The problem is that the code assumes that the resource will define at least one content type but doesn't check to make sure this is true.

@stevan - any thoughts on what we should do? At the very least we should provide a better error message, but I'm wondering if it might be better to try to work around this. Someone could create a resource class that manually sets the Content-Type header later in the response cycle, for example.