title: pig tagsource doesnt work with multiple files
body: <p>I have a Pig script that loads multiple input files.  When I want to get the input filename with each input record from the file <code>tagsource</code> or <code>tagfile</code>, it only pickups the first filename but includes the data from both files.</p>
  
  <p><strong>Example</strong>:</p>
  
  <pre><code>I_STREAM = LOAD '/tmp/' USING PigStorage(';','-tagFile') AS (filename, f1, f2, f3);
  DUMP I_STREAM; 
  </code></pre>
  
  <p><strong>Contents of Files</strong>:</p>
  
  <pre><code>/tmp$ cat ./file1.txt
  1;1;1 
  
  /tmp$ cat ./file2.txt
  2;2;2
  </code></pre>
  
  <p><strong>Current Output</strong>:</p>
  
  <pre><code>(file1.txt, 1,1,1)
  (file1.txt, 2,2,2)
  </code></pre>
  
  <p><strong>Expected Output</strong>:</p>
  
  <pre><code>(file1.txt, 1,1,1)
  (file2.txt, 2,2,2)
  </code></pre>