beater/api/profile: fix pprof content-type

[axw]

Currently the content-type we look for for pprof is application/x-protobuf, but we should be more specific.

The Go agent is sending application/x-protobuf; messageType=”perftools.profiles.Profile". Note the use of ” rather than ", presumably copy-pasta. We'll need to fix that, and then update the server to check for the messageType suffix too.