fix: use correct namespace for the MapReturnType

[opti]

That fixes the following error when map expressions are used:

NameError:
       uninitialized constant Aerospike::Exp::Map::MapReturnType

/aerospike-client-ruby/lib/aerospike/exp/exp_map.rb:479:in `get_value_type'
/aerospike-client-ruby/lib/aerospike/exp/exp_map.rb:294:in `get_by_key_range'

[khaf]

Thank you for your PR. I'm going to merge and release today, so if you have discovered other issues, you can mention them here so that I can take a look and potentially include them in the same release.

[opti]

Thanks @khaf! Here is one more fix to include that I've prepared earlier: #132

[opti]

I've just added one more fix to the #132 for BatchRead when multiple records with operations are used.