feat: add method property to responses (#123)

```console
# before: httpbingo lacks the "method" field but httpbin.org has it
$ curl -s https://httpbingo.org/anything | jq .method                                                                                                        9:31PM
null
$ curl -s https://httpbin.org/anything | jq .method
"GET"

# after: httpbingo has the method field!
$ curl -s http://0.0.0.0:8080/anything | jq .method
"GET"
```

This was mentioned in #6, but then not listed in #91

- I took the highly technical testing approach of adding a test for
`Method` anywhere in `handlers_test.go` that mentioned `Args`; do you
desire more tests? fewer tests?
- Anywhere else I should add this field that I didn't already?

For what it's worth, I discovered this when working on upgrading the
testing for our [httpsnippet
library](https://github.com/readmeio/httpsnippet); I wanted to use
`go-httpbin` but our tests expect the method field to be present in the
responses