#443 fix bug in new calculated offset which caused xref issue

src/UglyToad.PdfPig.Tests/Parser/Parts/FileHeaderParserTests.cs

@@ -170,7 +170,7 @@ three %PDF-1.6");
             var result = FileHeaderParser.Parse(scanner.scanner, scanner.bytes, false, log);
 
             Assert.Equal(0, scanner.scanner.CurrentPosition);
-            Assert.Equal(129, result.OffsetInFile);
+            Assert.Equal(128, result.OffsetInFile);
             Assert.Equal(1.1m, result.Version);
             Assert.Equal("PDF-1.1", result.VersionString);
         }

src/UglyToad.PdfPig/Parser/FileStructure/FileHeaderParser.cs

@@ -131,7 +131,7 @@
                         headerVersion = new HeaderVersion(
                             version,
                             content.Substring(afterCommentSymbolIndex, versionLength - 1),
-                            currentOffset + afterCommentSymbolIndex);
+                            currentOffset + actualIndex);
 
                         inputBytes.Seek(startPosition);