fixup! Ensure filter streams expose properties that makes sense

Author: nulltoken

LibGit2Sharp.Tests/FilterFixture.cs

@@ -358,8 +358,11 @@ public void FilterStreamsAreCoherent()
             {
                 Assert.False(input.CanWrite);
                 Assert.True(input.CanRead);
+                Assert.False(input.CanSeek);
+
                 Assert.False(output.CanRead);
                 Assert.True(output.CanWrite);
+                Assert.False(input.CanSeek);
 
                 return GitPassThrough;
             };

LibGit2Sharp/Core/GitBufReadStream.cs

@@ -34,5 +34,15 @@ private static long ConvertToLong(UIntPtr len)
 
             return (long)len.ToUInt64();
         }
+
+        public override long Seek(long offset, SeekOrigin loc)
+        {
+            throw new NotSupportedException();
+        }
+
+        public override bool CanSeek
+        {
+            get { return false; }
+        }
     }
 }