ValidateParameter does not apply default arrays correctly in queries

[TheSadlig]

Hi,

When setting default arrays in query parameters, the applied default value is a string, due to the fmt.Sprintf(%v here:
https://github.com/getkin/kin-openapi/blob/master/openapi3filter/validate_request.go#L159

For example, if you default is []string{"A", "B", "C"}
then input.Request.URL.Query()

• contains []string{"[A B C]"} (note that this is an array with ONE string)
• but should contain: []string{"A", "B", "C"}

The following unit test reproduces the issues.

var (
	StringArraySchemaWithDefault = &openapi3.SchemaRef{
		Value: &openapi3.Schema{
			Type:    &openapi3.Types{"array"},
			Items:   stringSchema,
			Default: []string{"A", "B", "C"},
		},
	}
)

func TestValidateRequestDefault(t *testing.T) {
	type testCase struct {
		name       string
		param      *openapi3.Parameter
		query      string
		wantQuery  map[string][]string
	}

	testCases := []testCase{
		{
			name: "String Array In Query",
			param: &openapi3.Parameter{
				Name: "param", In: "query", Style: "form", Explode: explode,
				Schema: StringArraySchemaWithDefault,
			},
			wantQuery: map[string][]string{
				"param": {
					"A",
					"B",
					"C",
				},
			},
		},
	}

	for _, tc := range testCases {
		t.Run(tc.name, func(t *testing.T) {
			info := &openapi3.Info{
				Title:   "MyAPI",
				Version: "0.1",
			}
			doc := &openapi3.T{OpenAPI: "3.0.0", Info: info, Paths: openapi3.NewPaths()}
			op := &openapi3.Operation{
				OperationID: "test",
				Parameters:  []*openapi3.ParameterRef{{Value: tc.param}},
				Responses:   openapi3.NewResponses(),
			}
			doc.AddOperation("/test", http.MethodGet, op)
			err := doc.Validate(context.Background())
			require.NoError(t, err)
			router, err := legacyrouter.NewRouter(doc)
			require.NoError(t, err)

			req, err := http.NewRequest(http.MethodGet, "http://test.org/test?"+tc.query, nil)
			route, pathParams, err := router.FindRoute(req)
			require.NoError(t, err)

			input := &RequestValidationInput{Request: req, PathParams: pathParams, Route: route}

			err = ValidateParameter(context.Background(), input, tc.param)
			require.NoError(t, err)

			for k, v := range tc.wantQuery {
				require.Equal(t, v, input.GetQueryParams()[k])
			}
		})
	}
}