fix Context.Next() - recheck len of handlers every iteration (#1745)

* fix Context.Next() - recheck len of handlers every iteration

* add tests when Context.reset() can be called inside of handler

TestEngineHandleContext
TestContextResetInHandler
TestRouterStaticFSFileNotFound

* Context.Next() - format to while style

context.go

@@ -105,8 +105,9 @@ func (c *Context) Handler() HandlerFunc {
 // See example in GitHub.
 func (c *Context) Next() {
 	c.index++
-	for s := int8(len(c.handlers)); c.index < s; c.index++ {
+	for c.index < int8(len(c.handlers)) {
 		c.handlers[c.index](c)
+		c.index++
 	}
 }