Fix deprecated fields

[vektah]

When answering introspection queries we were returning objects like this:

 {
          "kind": "OBJECT",
          "name": "Chatroom",
          "description": "",
          "fields": [
            {
              "name": "name",
              "description": "",
              "args": [
                
              ],
              "type": {
                "kind": "NON_NULL",
                "name": null,
                "ofType": {
                  "kind": "SCALAR",
                  "name": "String",
                  "ofType": null
                }
              },
              "isDeprecated": false,
              "deprecationReason": ""
            },
            {
              "name": "messages",
              "description": "",
              "args": [
                
              ],
              "type": {
                "kind": "NON_NULL",
                "name": null,
                "ofType": {
                  "kind": "LIST",
                  "name": null,
                  "ofType": {
                    "kind": "NON_NULL",
                    "name": null,
                    "ofType": {
                      "kind": "OBJECT",
                      "name": "Message",
                      "ofType": null
                    }
                  }
                }
              },
              "isDeprecated": false,
              "deprecationReason": ""
            }

deprecationReason: ""

This PR changes introspection to return null instead.

Fixes #436

I have:

• Added tests covering the bug / feature (see testing)

[ghost]

After this merge to master, my code is returning following error while playground is executing query IntrospectionQuery {.....}

panic: interface conversion: interface {} is func() bool, not bool

goroutine 181 [running]:
github/a/b/graph/exec.(*executionContext).___Field_isDeprecated(0xc00035ed10, 0xd2e440, 0xc0003bb9e0, 0xc0002cfa00, 0x0, 0x0, 0x0, 0xc00035c4c8, 0x0, 0x0)
        /go/src/github/a/b/graph/exec/generated.go:13776 +0x541
github/a/b/graph/exec.(*executionContext).___Field(0xc00035ed10, 0xd2e440, 0xc0003bb9e0, 0xc0002df140, 0xc, 0xc, 0xc00035c4c8, 0x0, 0xc000417748)
        /go/src/github/a/b/graph/exec/generated.go:13592 +0x9a6
github/a/b/graph/exec.(*executionContext).___Type_fields.func2.1(0xc00035ed10, 0xd2e440, 0xc0003bb9e0, 0xc0002cf780, 0xc0002df140, 0xc, 0xc, 0xc00035c480, 0x2, 0x2, ...)
        /go/src/github/a/b/graph/exec/generated.go:14390 +0x91
github/a/b/graph/exec.(*executionContext).___Type_fields.func2(0x1)
        /go/src/github/a/b/graph/exec/generated.go:14391 +0xb3
created by github/a/b/graph/exec.(*executionContext).___Type_fields
        /go/src/github/a/b/graph/exec/generated.go:14396 +0x6c4

[vektah]

You need to regenerate after updating gqlgen