bigdecimal specs for to_s - add specs for integer and exponent part

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).
Merge pull request #2 from mingulov/to_s

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