Fix media_types running under Deno with ESM

[kitsonk]

Fixes the importing of JSON. Requires a build of Deno that includes denoland/deno#1514 (e.g. current master).

Upgrading to a version of Deno that includes ESM will "break" media_types and the other patch and this one will "fix" it.

[ry]

With v0.2.7, I'm also getting an error which seems related:

assertEqual failed. actual = text/plain expected = text/yaml... FAILED
Error: actual: text/plain expected: text/yaml
    at assertEqual (/Users/rld/src/deno_std/testing/mod.ts:23:15)
    at serveFile (/Users/rld/src/deno_std/http/file_server_test.ts:20:9)

[kitsonk]

@ry I will take a look

[kitsonk]

@ry I cannot reproduce locally... off of master, against this PR:

test serveFile [2019-01-15 06:07:39] "GET /azure-pipelines.yml HTTP/1.1" 200
... ok

If there are things cached locally for some reason, it could throw this off. I will update the PR to bump the version of Deno.

[ry]

LGTM - thanks for looking - must be somebthng on my side.