Ruby: Use base64 encoding for  :pack_simple_arrays option (to_simple_value)

Uses base64 encoded strings as return value for to_simple_value, in case :pack_simple_arrays option is used.

Base64.strict_encode64 is available from ruby 1.9.2, it is needed, bacause Base64.encode64 fills the String with additional newlines at teh end or after 40 chars (default in http headers, but not wanted here).
The size is not the size of the string, it is the size of the container that you need to unpack the string. That's definitely not encoded anywhere anymore (by the way, the unit tests would have caught it, please update the unit tests).