fix(codec): Remove custom content-type (#104)

This removes custom content-types in favor of
just using `application/grpc`. There is some
confusion around the specification but most
grpc implementations ignore the `+` and
anything after.

tonic-examples/build.rs

@@ -2,4 +2,5 @@ fn main() {
     tonic_build::compile_protos("proto/helloworld/helloworld.proto").unwrap();
     tonic_build::compile_protos("proto/routeguide/route_guide.proto").unwrap();
     tonic_build::compile_protos("proto/echo/echo.proto").unwrap();
+    tonic_build::compile_protos("proto/google/pubsub/pubsub.proto").unwrap();
 }