Rob, you know more about DHT11/DHT22 than anyone I think.
Too much honor, I am only standing on the shoulders of giants**
Rob, the pointer you gave uses DS18B20 and I need to dig into that. It uses a library that hides the details of building the HTTP PUT format characters. Maybe that's good. I was hoping to understand the gritty details..
The solution seems to be in this "new" cosm lib - GitHub - blawson/PachubeArduino -
especially the cpp file - PachubeArduino/Cosm.cpp at master · blawson/PachubeArduino · GitHub -
it uses _client.print(dataToSend, DEC);
when dataToSend is a float/double, DEC will be interpreted as the number of decimals. // not too nice usage of DEC but OK
but most important, OP states it works/solves his problem of posting floats to cosm.
I have not confirmed it (no duino with ethernet nearby) but I expect you will within a day or so 
Let us know if it works.
** originally from Sir Isaac Newton IIRC