Do compress grpc-web responses

[bouk]

Related: #215

Motivation

grpc-web responses aren't compressed right now, but they should be.

Solution

Add exception for application/grpc-web and change exception filter to use starts_with so we match on any encoding format.

[bouk]

@jplatte can you take a look at this one?

[jplatte]

Is there a piece of documentation or article that you can link to for justification / proof that this is the right thing to do? I'm not really familiar with GRPC.