Remove the dependency of github.com/gorilla/context

[xuwei0455]

I found github.com/gorilla/context had been in maintenance mode, After some search of dependency for it.

I truly just find one place that uses the gorilla context, as follows:
https://github.com/gin-contrib/sessions/blob/master/sessions.go#L64

func Sessions(name string, store Store) gin.HandlerFunc {
	return func(c *gin.Context) {
		s := &session{name, c.Request, store, nil, false, c.Writer}
		c.Set(DefaultKey, s)
		defer context.Clear(c.Request)
		c.Next()
	}
}

even github.com/gorilla/sessionshad removed the dependency, so why we need to keep the dependency?

[maximerety]

There are two occurrences now, with the addition of SessionsMany.

This was an emulation of context from a time when it wasn't a part of the standard library yet (golang < 1.7).

But what's bothering me the most is that the session is actually stored in a gin.Context (c.Set(DefaultKey, s)), which is definitely not the context that is cleared by defer context.Clear(c.Request).

Either I'm missing something or this does not make sense.

[GwynethLlewelyn]

Ah! That certainly explains why I cannot clear a session, no matter how hard I try. It does not make any sense to me, either... see also #89 — I believe it might be related to this.

[KeiichiHirobe]

@maximerety
FYI: #151

[marvin-min]

Got the same issue. Why 151 didn't get merged?