Prevent panic in Context.GetQuery() when there is no Request

I have an endpoint that uses an optional query parameter via DefaultQuery(). 
In one unit test case I want to test the route with that parameter.
I write that test as
```go
	w := httptest.NewRecorder()
	c, _ := gin.CreateTestContext(w)
	routeHandler(c)
```
And this panics when routeHandler() calls c.DefaultQuery() because c.Request == nil.

context.go

@@ -416,7 +416,11 @@ func (c *Context) QueryArray(key string) []string {
 
 func (c *Context) initQueryCache() {
 	if c.queryCache == nil {
-		c.queryCache = c.Request.URL.Query()
+		if c.Request != nil {
+			c.queryCache = c.Request.URL.Query()
+		} else {
+			c.queryCache = url.Values{}
+		}
 	}
 }