bigdecimal specs for to_s - add specs for spaces at integer and exponent parts

bigdecimal to_s spec for 'inserts a space every n chars, if integer n is supplied' - add checks for integer part of the number (it must have a space every n chars), and check that the exponent part does not have spaces (it is the current behaviour for Ruby MRI and Rubinius).

This behaviour was not specified before so it might be that some implementations will have different behaviour.
The added specs are valid for Ruby 1.8.7, 2.0 and Rubinius 2.2.1.