http: gRPC-web filter does not clear the route cache

*Title*: *gRPC-web filter does not clear the route cache*

*Description*:
>`source/extensions/filters/http/grpc_web/grpc_web_filter.cc` checks if the incoming request is a gRPC-web request and transforms it to gRPC proper. The transformation involves changing the content type to `application/grpc`. Unlike the JSON transcoding filter, gRPC-web one does not however clear the route cache making it harder to route the "upgraded" request the same way one would route a native gRPC request.

*Envoy version*: `master @ 97d259d43a4f9b7c6d798804b4b2c9ffbd2d285d` (02/11/2019)

*Repro steps*:
Define a route explicitly matching gRPC traffic:
```json
...
"match": {
"prefix": "/",
"grpc": {}
},
"route": {
"cluster": "lo_svc_grpc",
"max_grpc_timeout": {
"seconds": 0
}
},
{
"match": {
"prefix": "/"
},
"route": {
"cluster": "lo_svc_http",
"timeout": {
"seconds": 60
}
}
}
...
```
Define the http filters in the following order:
```json
...
"http_filters": [
{
"name": "envoy.grpc_web"
},
{
"name": "envoy.router"
}]
...
```

Send a gRPC-web request through Envoy

Expected: the request is upgraded to gRPC proper and routed to `lo_svc_grpc`
Actual: the request is upgraded to gRPC proper and routed to `lo_svc_http` (the cached route)

Why is this a bug?

1) JSON transcoding filter clears the cache and supports such routing - consistency issue.
2) The documentation states the filters are executed in order of their definition, since the router was defined as the last one, it should pick up the changes made by the previous filters.
3) The routing supports matching on `gRPC` requests and when the gRPC-web request reaches the router, it is a `gRPC` request at that point and should be routed as such.

*Workaround*:
Insert a route matching the initial request on `gRPC-web` explicitly:
```json
{
"match": {
"prefix": "/",
"headers": [
{
"name": "content-type",
"prefix_match": "application/grpc-web"
}
]
},
"route": {
"cluster": "lo_svc_grpc",
"max_grpc_timeout": {
"seconds": 0
}
}
},
```

*Proposed fix*:
```c++
decoder_callbacks_->clearRouteCache();
```
after the line `https://github.com/envoyproxy/envoy/blob/master/source/extensions/filters/http/grpc_web/grpc_web_filter.cc#L60`