Add CopyCoder#null_string to gc_mark and #to_h .
Add PG::TypeMapAllStrings and use it as default typemap, when no explicit
typemap was set. This allowes PG::CopyCoder to work without typemap, too.
Allow inline checks for string capacity. This removes the bottleneck in the COPY
decoder, when doing the check for each character.
Add typecasting to Connection#get_copy_data.