fix buffering middleware

[ldez]

What does this PR do?

fix buffering middleware

Motivation

DEBU[2019-09-02T15:52:55+02:00] Configuration received from provider file: {"http":{"routers":{"router0":{"entryPoints":["http"],"middlewares":["test-stripprefix"],"service":"service-foo","rule":"Path(`/bar`)"}},"middlewares":{"test-stripprefix":{"buffering":{"maxRequestBodyBytes":250000}}},"services":{"service-foo":{"loadBalancer":{"servers":[{"url":"http://127.0.0.1:3030"}],"passHostHeader":false}}}},"tcp":{},"tls":{}}  providerName=file
DEBU[2019-09-02T15:52:55+02:00] Creating middleware                           middlewareName=pipelining entryPointName=http routerName=router0@file serviceName=service-foo middlewareType=Pipelining
DEBU[2019-09-02T15:52:55+02:00] Creating load-balancer                        entryPointName=http routerName=router0@file serviceName=service-foo
DEBU[2019-09-02T15:52:55+02:00] Creating server 0 http://127.0.0.1:3030       routerName=router0@file serviceName=service-foo serverName=0 entryPointName=http
DEBU[2019-09-02T15:52:55+02:00] Added outgoing tracing middleware service-foo  routerName=router0@file entryPointName=http middlewareName=tracing middlewareType=TracingForwarder
ERRO[2019-09-02T15:52:55+02:00] Error in Go routine: runtime error: invalid memory address or nil pointer dereference 
ERRO[2019-09-02T15:52:55+02:00] Stack: goroutine 16 [running]:
runtime/debug.Stack(0xc000124000, 0x24bdd12, 0x17)
	/usr/local/go/src/runtime/debug/stack.go:24 +0x9d
github.com/containous/traefik/v2/pkg/safe.defaultRecoverGoroutine(0x208cf40, 0x3e64dc0)
	/go/src/github.com/containous/traefik/pkg/safe/routine.go:159 +0x9d
github.com/containous/traefik/v2/pkg/safe.GoWithRecover.func1.1(0x2598a00)
	/go/src/github.com/containous/traefik/pkg/safe/routine.go:150 +0x57
panic(0x208cf40, 0x3e64dc0)
	/usr/local/go/src/runtime/panic.go:679 +0x1b2
github.com/containous/traefik/v2/pkg/server/middleware.(*Builder).buildConstructor(0xc0001704e0, 0x290db80, 0xc0004c6d80, 0xc0002760a0, 0x15, 0xc0004c6d80, 0x0, 0x0)
	/go/src/github.com/containous/traefik/pkg/server/middleware/middlewares.go:126 +0x130
github.com/containous/traefik/v2/pkg/server/middleware.(*Builder).BuildChain.func1(0x28ae280, 0xc0004c6d50, 0x28ae280, 0xc0004c6d50, 0x0, 0x0)
	/go/src/github.com/containous/traefik/pkg/server/middleware/middlewares.go:73 +0x1e0
github.com/containous/alice.Chain.Then(0xc00014cbc0, 0x2, 0x2, 0x28ae000, 0xc00014cba0, 0x1, 0x1, 0x1, 0x0)
	/go/pkg/mod/github.com/containous/alice@v0.0.0-20181107144136-d83ebdd94cbd/chain.go:51 +0x8f
github.com/containous/traefik/v2/pkg/server/router.(*Manager).buildHTTPHandler(0xc000477b78, 0x290db80, 0xc0004c69f0, 0xc0004c65d0, 0xc000447a40, 0xc, 0xc0004c69f0, 0x1f72800, 0xc0005635c8, 0xc0005635c8)
	/go/src/github.com/containous/traefik/pkg/server/router/router.go:167 +0x576
github.com/containous/traefik/v2/pkg/server/router.(*Manager).buildRouterHandler(0xc000477b78, 0x290db80, 0xc0004c69f0, 0xc000447a40, 0xc, 0xc0004c65d0, 0xc0004c69f0, 0x2440980, 0xc0004841c0, 0xc0000236d0)
	/go/src/github.com/containous/traefik/pkg/server/router/router.go:127 +0xd2
github.com/containous/traefik/v2/pkg/server/router.(*Manager).buildEntryPointHandler(0xc000477b78, 0x290db80, 0xc0004c68a0, 0xc0004c6780, 0x1, 0x290db80, 0xc0004c68a0, 0xc0004c6750)
	/go/src/github.com/containous/traefik/pkg/server/router/router.go:97 +0x22e
github.com/containous/traefik/v2/pkg/server/router.(*Manager).BuildHandlers(0xc000477b78, 0x290db00, 0xc0000d2008, 0xc000170460, 0x2, 0x2, 0x0, 0x0)
	/go/src/github.com/containous/traefik/pkg/server/router/router.go:65 +0x1f4
github.com/containous/traefik/v2/pkg/server.(*Server).createHTTPHandlers(0xc000034340, 0x290db00, 0xc0000d2008, 0xc0004c6570, 0xc000170460, 0x2, 0x2, 0x2, 0xc000503fb0)
	/go/src/github.com/containous/traefik/pkg/server/server_configuration.go:105 +0x2eb
github.com/containous/traefik/v2/pkg/server.(*Server).loadConfigurationTCP(0xc000034340, 0xc0003dfda0, 0x3ead580)
	/go/src/github.com/containous/traefik/pkg/server/server_configuration.go:78 +0x283
github.com/containous/traefik/v2/pkg/server.(*Server).loadConfiguration(0xc000034340, 0x24977d4, 0x4, 0xc0004ca2a0)
	/go/src/github.com/containous/traefik/pkg/server/server_configuration.go:41 +0x11f
github.com/containous/traefik/v2/pkg/server.(*Server).listenConfigurations(0xc000034340, 0xc0001b22a0)
	/go/src/github.com/containous/traefik/pkg/server/server_configuration.go:244 +0x51
github.com/containous/traefik/v2/pkg/server.(*Server).Start.func3(0xc0001b22a0)
	/go/src/github.com/containous/traefik/pkg/server/server.go:172 +0x34
github.com/containous/traefik/v2/pkg/safe.(*Pool).Go.func1()
	/go/src/github.com/containous/traefik/pkg/safe/routine.go:90 +0x7b
github.com/containous/traefik/v2/pkg/safe.GoWithRecover.func1(0x2598a00, 0xc00026cff0)
	/go/src/github.com/containous/traefik/pkg/safe/routine.go:153 +0x57
created by github.com/containous/traefik/v2/pkg/safe.GoWithRecover
	/go/src/github.com/containous/traefik/pkg/safe/routine.go:147 +0x49 

More

• Added/updated tests
• Added/updated documentation

[juliens]

LGTM

[jbdoumenjou]

LGTM