Fix custom options behaviour
diff --git a/src/ProtoGen/SourceGeneratorBase.cs b/src/ProtoGen/SourceGeneratorBase.cs
index dea9def..1604f11 100644
--- a/src/ProtoGen/SourceGeneratorBase.cs
+++ b/src/ProtoGen/SourceGeneratorBase.cs
@@ -31,7 +31,7 @@
     }
 
     internal static string GetFieldConstantName(FieldDescriptor field) {
-      return NameHelpers.UnderscoresToPascalCase(GetFieldName(field)) + "FieldNumber";
+      return field.CSharpOptions.PropertyName + "FieldNumber";
     }
 
     private static string ToCSharpName(string name, FileDescriptor file) {