Two tiny updates to keep mongo.cr on par with latest Crystal version

src/bson/bson_error.cr

@@ -1,6 +1,9 @@
 class BSON
   class BSONError < Exception
     getter domain, code, detail
+  	@domain : UInt32
+  	@code : UInt32
+  	@detail : String
 
     def initialize(bson_error)
       @domain = bson_error.value.domain
@@ -9,4 +12,4 @@ class BSON
       super("Domain: #{@domain}, code: #{@code}, #{@detail}")
     end
   end
-end
+end

src/mongo/read_prefs.cr

@@ -3,7 +3,7 @@ class Mongo::ReadPrefs
     raise "invalid handle" unless @handle
   end
 
-  def initialize(mode = LibMongoC::ReadMode::PRIMARY : LibMongoC::ReadMode)
+  def initialize(mode : LibMongoC::ReadMode::PRIMARY = LibMongoC::ReadMode)
     initialize LibMongoC.read_prefs_new(mode)
   end