Moved sending of initial metadata from server into server handler

src/ruby/lib/grpc/generic/active_call.rb

src/ruby/lib/grpc/generic/bidi_call.rb

src/ruby/lib/grpc/generic/rpc_desc.rb

src/ruby/lib/grpc/generic/rpc_server.rb

src/ruby/spec/generic/active_call_spec.rb

src/ruby/spec/generic/rpc_desc_spec.rb