Ensure toArrayStream always returns an ArrayStream (#61)

Throw in `toArrayStream` when passing a non-`ArrayStream` stream,
to ensure that it always returns an `ArrayStream`.

When converting between stream types, call `readToEnd` first.

Author: twiss

lib/streams.js

@@ -25,7 +25,11 @@ function toStream(input) {
  * @returns {ArrayStream} Converted data
  */
 function toArrayStream(input) {
-  if (isStream(input)) {
+  const streamType = isStream(input);
+  if (streamType) {
+    if (streamType !== 'array') {
+      throw new Error("Can't convert Stream to ArrayStream here, call `readToEnd` first");
+    }
     return input;
   }
   const stream = new ArrayStream();