The documentation for oauth2.Exchange at https://godoc.org/golang.org/x/oauth2#Config.Exchange says:
The HTTP client to use is derived from the context. If a client is not provided via the context, http.DefaultClient is used.
Now I have created a http.Client with my own settings. How do I get the oauth2.Exchange function to use it?
oauth2.Exchange的文档，位于 https://godoc.org/golang.org/x/oauth2#Config.Exchange 说：
要使用的HTTP客户端是从上下文派生的。 如果没有通过上下文提供客户端，则使用http.DefaultClient。
现在，我已经使用自己的设置创建了一个http.Client。 如何获得oauth2.Exchange函数以使用它？
The oauth2 package defines the following variable:
var HTTPClient internal.ContextKeyHTTPClient is the context key to use with golang.org/x/net/context's WithValue function to associate an *http.Client value with a context.
Use the above variable as the context key when constructing the context to pass to Exchange:
微信扫一扫client := &http.Client{
// TODO
}
parent := oauth2.NoContext()
ctx := context.WithValue(parent, oauth2.HTTPClient, client)
tkn, err := c.Exchange(ctx, code)