[Firebird] Not-null blobs and remove table names from set clause in update statement

Not-null blob values were inserting null values and failing as described in #430. I basically copied the changes that were made in @47bcdcf866a6c2303d6e64cf83b8441affccf706 for the DB2 adapter.

Firebird can't have the table name in the set clause of an update statement. I added the quote_table_name_for_assignment override used in the SQLite3 adapter. (https://github.com/jruby/activerecord-jdbc-adapter/blob/master/lib/arjdbc/sqlite3/adapter.rb#L226-L228)