Demonstrate poor handling of cyclic datastructures

This PR is not meant to ever be merged. This is a demonstration of some
broken behavior around handling datastructures with cyclic dependencies.
The problem is if we provide a struct to `MarshalQueryWithOptions()` and
that struct has cyclic fields, even if not populated, and even if we
provide a Field that limits the output to a set depth,
`MarshalQueryWithOptions()` will still recurse infinitely and cause a
panic due to exhausting the stack.

This is a big problem because goql is a GraphQL library, and GraphQL is
for querying graph datastructures, and datastructures in graphs will
very commonly have cyclic relationships because graphs have cycles :)

The correct behavior would be to:

1. If no Fields are provided, do not recursively walk into fields of
   structs that've already been walked. This is safe to do because
   GraphQL already doesn't allow infinite expansion; if you want
   relationships, even recursive ones, to a particular depth then GQL
   will force you to specify that in your query. Thus we can assume that
   you'll have to specify that to `goql` via a `Fields` param.
2. If `Fields` *are* provided, we can recursively walk already walked
   fields, but only to the depth specified in those `Fields`.

Future work will have to be done to get this behavior to work.

query_test.go

@@ -7,6 +7,16 @@ import (
 	"github.com/pmezard/go-difflib/difflib"
 )
 
+type CyclicParent struct {
+	Name  string
+	Child *CyclicChild
+}
+
+type CyclicChild struct {
+	Name   string
+	Parent *CyclicParent
+}
+
 // TestMarshalQuery tests the MarshalQuery function.
 func TestMarshalQuery(t *testing.T) {
 	tt := []struct {
@@ -321,6 +331,53 @@ differentField
 testQuery {
 overrideName
 }
+}`,
+		},
+		{
+			Name: "CyclicStructuresWithFields",
+			Input: struct {
+				TestQuery struct {
+					Parent *CyclicParent
+				}
+			}{},
+			Fields: Fields{
+				"parent": Fields{
+					"name": true,
+					"child": Fields{
+						"name": true,
+					},
+				},
+			},
+			Option: OptFallbackJSONTag,
+			ExpectedOutput: `query {
+testQuery {
+parent {
+name
+child {
+name
+}
+}
+}
+}`,
+		},
+		{
+			Name: "CyclicStructuresWithoutFields",
+			Input: struct {
+				TestQuery struct {
+					Parent *CyclicParent
+				}
+			}{},
+			Fields: nil,
+			Option: OptFallbackJSONTag,
+			ExpectedOutput: `query {
+testQuery {
+parent {
+name
+child {
+name
+}
+}
+}
 }`,
 		},
 	}