test coverage and fix bugs with ascii 85 filter

src/UglyToad.Pdf.Tests/Filters/Ascii85FilterTests.cs

@@ -1,5 +1,6 @@
 namespace UglyToad.Pdf.Tests.Filters
 {
+    using System;
     using System.Text;
     using ContentStream;
     using Pdf.Filters;
@@ -29,6 +30,28 @@ O<DJ+*.@<*K0@<6L(Df-\0Ec5e;DffZ(EZee.Bl.9pF""AGXBPCsi + DGm >@3BB / F * &OCAfu2
                 text);
         }
 
+        [Fact]
+        public void ReplacesZWithEmptyBytes()
+        {
+            var bytes = Encoding.ASCII.GetBytes("9jqo^zBlbD-");
+
+            var result = filter.Decode(bytes, new PdfDictionary(), 1);
+
+            var text = Encoding.ASCII.GetString(result);
+
+            Assert.Equal("Man \0\0\0\0is d", text);
+        }
+
+        [Fact]
+        public void ZInMiddleOf5CharacterSequenceThrows()
+        {
+            var bytes = Encoding.ASCII.GetBytes("qjzqo^");
+
+            Action action = () => filter.Decode(bytes, new PdfDictionary(), 0);
+
+            Assert.Throws<InvalidOperationException>(action);
+        }
+
         private const string PdfContent = @"1 0 obj
 << /Length 568 >>
 stream
@@ -95,5 +118,7 @@ Li?EZek1DKKT1F`2DD/TboKAKY](@:s.m/h%oBC'mC/$>""*cF*)G6@;Q?_DIdZpC&";
 
             Assert.Equal(PdfContent.Replace("\r\n", "\n"), text);
         }
+
+
     }
 }

src/UglyToad.Pdf.Tests/Filters/AsciiHexDecodeFilterTests.cs

@@ -1,7 +1,6 @@
 namespace UglyToad.Pdf.Tests.Filters
 {
     using System;
-    using System.Net.NetworkInformation;
     using System.Text;
     using ContentStream;
     using Pdf.Filters;

src/UglyToad.Pdf/Filters/Ascii85Filter.cs

@@ -65,12 +65,12 @@
                     {
                         if (index > 0)
                         {
-                            // Should I throw here?
+                            throw new InvalidOperationException("Encountered z within a 5 character block");
                         }
 
                         for (int j = 0; j < 4; j++)
                         {
-                            writer.Write(0);
+                            writer.Write((byte)0);
                         }
 
                         index = 0;