Add multiple Accept headers to Manifest query

[cquon]

Resolves: #190

Previously:

$ reg manifest alpine
INFO[0000] domain: docker.io
INFO[0000] server address: https://registry-1.docker.io
{
   "schemaVersion": 2,
   "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
   "config": {
     "mediaType": "application/vnd.docker.container.image.v1+json",
     "size": 1512,
     "digest": "sha256:055936d3920576da37aa9bc460d70c5f212028bda1c08c0879aedf03d7a66ea1"
   },
   "layers": [
     {
       "mediaType": "application/vnd.docker.image.rootfs.diff.tar.gzip",
       "size": 2757034,
       "digest": "sha256:e7c96db7181be991f19a9fb6975cdbbd73c65f4a2681348e63a141a2192a5f10"
     }
   ]
 }

After change:

$ reg manifest alpine
INFO[0000] domain: docker.io
INFO[0000] server address: https://registry-1.docker.io
{
   "manifests": [
     {
       "digest": "sha256:bf1684a6e3676389ec861c602e97f27b03f14178e5bc3f70dce198f9f160cce9",
       "mediaType": "application\/vnd.docker.distribution.manifest.v2+json",
       "platform": {
         "architecture": "amd64",
         "os": "linux"
       },
       "size": 528
     },
     {
       "digest": "sha256:fc6ad107f85e5210d7cbbdb00b6af0b8b1c94a2b0cecc77db19e853b87cd549e",
       "mediaType": "application\/vnd.docker.distribution.manifest.v2+json",
       "platform": {
         "architecture": "arm",
         "os": "linux",
         "variant": "v6"
       },
       "size": 528
     },
     {
       "digest": "sha256:f6d15ec5c7cf08079309c59f59ff1e092eb9a678ab891257b1d2b118e7aecc2b",
       "mediaType": "application\/vnd.docker.distribution.manifest.v2+json",
       "platform": {
         "architecture": "arm",
         "os": "linux",
         "variant": "v7"
       },
       "size": 528
     },
     {
       "digest": "sha256:9d2b3c9ba598ccd7c4aa49e5ad5b4cf47fb882adc3f5955d5e160b07f26a68ee",
       "mediaType": "application\/vnd.docker.distribution.manifest.v2+json",
       "platform": {
         "architecture": "arm64",
         "os": "linux",
         "variant": "v8"
       },
       "size": 528
     },
     {
       "digest": "sha256:1be52adf0390937540f21f66fb8dc4c155b3a0325e380001be08fd5923359e81",
       "mediaType": "application\/vnd.docker.distribution.manifest.v2+json",
       "platform": {
         "architecture": "386",
         "os": "linux"
       },
       "size": 528
     },
     {
       "digest": "sha256:27ca8e336361fa1e0f6efc92f88d3f2a98e0da8ce422bf3230271d576a1f50e7",
       "mediaType": "application\/vnd.docker.distribution.manifest.v2+json",
       "platform": {
         "architecture": "ppc64le",
         "os": "linux"
       },
       "size": 528
     },
     {
       "digest": "sha256:d438d3b6a72b602b70bd259ebfb344e388d8809c5abf691f6de397de8c9e4572",
       "mediaType": "application\/vnd.docker.distribution.manifest.v2+json",
       "platform": {
         "architecture": "s390x",
         "os": "linux"
       },
       "size": 528
     }
   ],
   "mediaType": "application\/vnd.docker.distribution.manifest.list.v2+json",
   "schemaVersion": 2
 }

[jose-bigio]

LGTM