Allow Multipart requests with parts of unknown length

I'm not sure if it's allowed by the http specification, but anyway http error `411 Length Required` is much clearer than an exception like:

	retrofit.RetrofitError: java.net.ProtocolException: expected 43 bytes but received 131424
	            at retrofit.RestAdapter$RestHandler.invokeRequest(RestAdapter.java:420)
	            at retrofit.RestAdapter$RestHandler.access$100(RestAdapter.java:262)
	            at retrofit.RestAdapter$RestHandler$1.call(RestAdapter.java:305)
	            at retrofit.RestAdapter$RestHandler$1.call(RestAdapter.java:303)
	            at retrofit.RestAdapter$RxSupport$1.onSubscribe(RestAdapter.java:247)