fixnum and bignum deprecated in Ruby>=2.4

[boutil]

Hi,

Running the tests in Debian (ruby2.7.1p83), I noticed two warnings and one test failure due to the deprecation of Fixnum and Bignum in favor of Integer since ruby 2.4(?).

In the first chunk, I removed the line containing Bignum, because it is exactly the same as the previous one, with Integer instead.

In the second chunk, I just replaced Fixnum by Integer

In the third one, I had to replace assert_equal by assert_match because the output of the :generic_class_writer was outputing two more commented lines about the validity and encoding of the string, which is ignored here for now.

Cheers

[marcandre]

Thanks for the PR. I'm ok with the changes, except about ignoring the two more commented lines about the validity and encoding of the string... This should ideally be working in all versions of Ruby...