title: iText doesn't process correct signature fields on strange situations
body: <p>I have a pdf and i'm processing it with itext 5.4.4 (i have tried the same with 5.5.5 and i got the same errors), while processing the file i try to verify if all signatures cover the full document and verify it with this code:</p>
  
  <pre><code>    boolean resp = false;
      InputStream inputStream = null;
      PdfReaderversionReader = null;
      PdfReader originalReader = null;
      String signatureName = "Signature1";
  ... loading orginal pdf and signature names ...
      try {
          inputStream = reader.getAcroFields().extractRevision(signatureName);
          versionReader = new PdfReader(inputStream);
      } catch (IOException e) {
          log.warn("unable to get revision for signature FIELD ", e);
      }
      //After that i have tried to use the actual one, but it still fails...
      if (versionReader == null) {
          versionReader = originalReader;
      }
      resp = versionReader.getAcroFields().signatureCoversWholeDocument(signatureName);
  </code></pre>
  
  <p>My first trouble happens while the creating the versionReader it fails on parsing bytes.
  Opening with rups the pdf lokks like have two signature fields tags with the same field name.
  - The first one contains byte range /ByteRange: [0, 160, 9634, 121571]<br>
  - and the second one /ByteRange: [0, 131726, 1131728, 3904]
  iText just recover the first of them and after that fails.
  while i was debugging code i have found that in method</p>
  
  <pre><code>com.itextpdf.text.pdf.AcroFields.fill()
  
  in com.itextpdf.text.pdf.AcroFields line 241 this code
  
  if (fields.containsKey(name))
    continue;
  </code></pre>
  
  <p>so it's clearly discarding this information, i don't know but it's possible iText has a bug? or i'm doing something wrong while reading the pdf file?
  The point is Adobe Acrobat reader validate all signatures without problems...</p>
  
  <p>This is the <a href="https://drive.google.com/file/d/0BwBTJ2YupEtaaWo5WmhWbm0zcUU/view?usp=sharing" rel="nofollow">PDF</a> with the problem.</p>
  
  <p>All help is well received, thank you in advance.</p>