GH #585: Adding a pre-dispatch sanity check for supported HTTP method:

It should not be possible to cause an internal server error by sending
an invalid HTTP method. Dancer 2 should return a 405 "Method Not Allowed"
instead.

We now inspect the environment hash to know whether it is possible to
build a request from the given parameters. The issue that spawned this
was being passed an unsupported method.

We keep a hash of supported methods in Dancer2::Core::Types available from
everywhere.
Merge branch 'method_not_found'
reflect changes
Fix typo in dsl halt subtest name
Wrap 'pass' keyword in Return::MultiLevel logic so it returns when executed.

The pass keyword is documented as "returning immediately" (like forward and halt).
Move the handling of 'pass' to the context object to wrap the Return::MultiLevel
logic around it to effect that return.

Adds a basic test.
Ensure send_error returns from route when executed (as documented).

Resolves #570.
Ensure send_file returns from route handler immediately as documented.

Also cleaned up the use of send_file within apps with prefixes (prefixes
get removed when looking for the file on disk).
Cleanup DSL; s/self->app->context/self->context/
Reflect changes
Merge branch 'pr/send_error_return'
GH #587: Port Serializer::Mutable

Serializer::Mutable is available on D1 but not D2.
No more!

The following helper DSL keywords would shift and take app, even
though it wasn't necessary, and it broke our ability to work with
them (whether we were "supposed to" work with them or not). Because
it isn't actually using the app, we replaced them with "shift":

* from_json
* to_json
* from_yaml
* to_yaml
* from_dumper
* to_dumper
reflect changes
Merge branch 'pedrob/mutable'
update version
Merge branch 'release/0.140900_01'
stable, we hope
Merge branch 'release/0.14100'
Support all JSON configuration options

As noted in issue #512 the current JSON serializer ignored most configuration
options, so pass them all through.
Tests for passing config options to JSON serializer.
Reflect changes; addes @nichtich as a contributor.