Fix TextDecoderStream's behavior when processing the end of stream

This change fixes TextDecoderStream's "flush and enqueue" algorithm to handle the end of the stream in the same way as a call to TextDecoder's decode() method with no parameters.

Tests: web-platform-tests/wpt#29481.

Fixes #263.

encoding.bs

@@ -1810,23 +1810,35 @@ steps:
  <li><p>Let <var>output</var> be the <a for=/>I/O queue</a> of scalar values
  « <a>end-of-queue</a> ».
 
- <li><p>Let <var>result</var> be the result of <a>processing an item</a> with <a>end-of-queue</a>,
- <var>decoder</var>'s <a for=TextDecoderCommon>decoder</a>, <var>decoder</var>'s
- <a for=TextDecoderCommon>I/O queue</a>, <var>output</var>, and <var>decoder</var>'s
- <a for=TextDecoderCommon>error mode</a>.
-
  <li>
-  <p>If <var>result</var> is <a>finished</a>, then:
+  <p>While true:
 
   <ol>
-   <li><p>Let <var>outputChunk</var> be the result of running <a>serialize I/O queue</a> with
-   <var>decoder</var> and <var>output</var>.
+   <li><p>Let <var>item</var> be the result of <a>reading</a> from <var>decoder</var>'s
+   <a for=TextDecoderCommon>I/O queue</a>.
 
-   <li><p>If <var>outputChunk</var> is non-empty, then <a for=TransformStream>enqueue</a>
-   <var>outputChunk</var> in <var>decoder</var>'s <a for=GenericTransformStream>transform</a>.
-  </ol>
+   <li><p>Let <var>result</var> be the result of <a>processing an item</a> with <var>item</var>,
+   <var>decoder</var>'s <a for=TextDecoderCommon>decoder</a>, <var>decoder</var>'s
+   <a for=TextDecoderCommon>I/O queue</a>, <var>output</var>, and <var>decoder</var>'s
+   <a for=TextDecoderCommon>error mode</a>.
 
- <li><p>Otherwise, <a>throw</a> a {{TypeError}}.
+   <li>
+    <p>If <var>result</var> is <a>finished</a>, then:
+
+    <ol>
+     <li><p>Let <var>outputChunk</var> be the result of running <a>serialize I/O queue</a> with
+     <var>decoder</var> and <var>output</var>.
+
+     <li><p>If <var>outputChunk</var> is non-empty, then <a for=TransformStream>enqueue</a>
+     <var>outputChunk</var> in <var>decoder</var>'s <a for=GenericTransformStream>transform</a>.
+
+     <li><p>Return.
+    </ol>
+   </li>
+
+   <li><p>Otherwise, if <var>result</var> is <a>error</a>, <a>throw</a> a {{TypeError}}.
+  </ol>
+ </li>
 </ol>