(piqi-rpc) Less strict validation of HTTP request
(piqi-rpc) Less strict validation of HTTP request

- Allow "Content-Type" header value to be defined, but empty
- Recognize empty body for content type other than "application/x-protobuf" as
  empty input parameter

This allows easier HTTP request construction on the client side especially with
those HTTP libraries that require "Content-Type" header to be defined for each
POST request even if the body is empty.

alavrik/piqi-rpc,linearregression/piqi-rpc