…context for Go>=1.7

*gin.Context implements standard context.Context methods, but always
returns data as context is still valid. Since Go 1.7, http.Request now
contains a context.Context object, which can be controlled by the
http.Server to indicates that the context is now closed, and persue of
request should be canceled.

This implements the propagation of http.Request context methods inside
gin.Context to have HTTP context cancelation information at gin.Context
level.

Signed-off-by: Romain Beuque <romain.beuque@corp.ovh.com>