map missing character codes directly #44

previously if no matching unicode was found for a character code we would return a null letter. instead we now map from the character code directly to a character. this seems to work for most documents, except where there are ligatures, e.g. fi or ff, but is still better than not returning anything.

src/UglyToad.PdfPig.Tests/Integration/LaTexTests.cs

@@ -24,7 +24,7 @@
 
                 var page2 = document.GetPage(2);
 
-                Assert.Contains("isθc={θc1,θc2,...,θcn},", page2.Text);
+                Assert.Contains("is~θc={θc1,θc2,...,θcn},", page2.Text);
             }
         }