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).
Now, we are implementing a JSON api, it is not ONE integer, it is a 7 byte string: "size: 5"

But more important:
In order to demarshal properly, you have to look into the pack code anyways, to get the content size (int8,int32, float32, whatever) and endianess right.

Hard-coding the content type in each language (and implementation) will break a lot in case the (base-)type changes, resulting in a lot of errors in the code of API users.