chore: Improve implicit Content-Length failures

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

See-Also: #560

lib/http/request/body.rb

@@ -17,14 +17,13 @@ def initialize(source)
       def size
         if @source.is_a?(String)
           @source.bytesize
-        elsif @source.respond_to?(:read)
-          raise RequestError, "IO object must respond to #size" unless @source.respond_to?(:size)
-
+        elsif @source.respond_to?(:read) && @source.respond_to?(:size)
           @source.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