chore: Improve implicit Content-Length failures

Suggest setting `Content-Length` header manually in case we can't guess
body size.

See-Also: #560
Co-Authored-By: @savonarola <ilya.averyanov@gmail.com>

lib/http/request/body.rb

@@ -24,7 +24,8 @@ def size
         elsif @source.nil?
           0
         else
-          raise RequestError, "cannot determine size of body: #{@source.inspect}"
+          raise RequestError, "cannot determine size of body: #{@source.inspect}, " \
+            "please set 'Content-Length' header explicitly"
         end
       end