Fails to load in JRuby: RangeError (bignum too big to convert into `long')

This module expects in multiple locations that it can convert the special numbers 0 and 2^64-1 into a time stamp. This does not work in JRuby, where it uses a signed long to work on times.

```
irb(main):002:0> require "ssh_data"
Traceback (most recent call last):
16: from org/jruby/RubyKernel.java:1052:in `load'
15: from /usr/share/jruby/bin/irb:11:in `'
14: from org/jruby/RubyKernel.java:1237:in `catch'
13: from org/jruby/RubyKernel.java:1237:in `catch'
12: from org/jruby/RubyKernel.java:1507:in `loop'
11: from org/jruby/RubyKernel.java:1091:in `eval'
10: from (irb):2:in `evaluate'
9: from /usr/share/jruby/lib/ruby/stdlib/rubygems/core_ext/kernel_require.rb:160:in `require'
8: from org/jruby/RubyKernel.java:1017:in `require'
7: from /var/lib/puppetserver/jruby-gems/gems/ssh_data-1.3.0/lib/ssh_data.rb:33:in `'
6: from /usr/share/jruby/lib/ruby/stdlib/rubygems/core_ext/kernel_require.rb:85:in `require'
5: from org/jruby/RubyKernel.java:1017:in `require'
4: from /var/lib/puppetserver/jruby-gems/gems/ssh_data-1.3.0/lib/ssh_data/certificate.rb:4:in `'
3: from /var/lib/puppetserver/jruby-gems/gems/ssh_data-1.3.0/lib/ssh_data/certificate.rb:5:in `'
2: from /var/lib/puppetserver/jruby-gems/gems/ssh_data-1.3.0/lib/ssh_data/certificate.rb:8:in `'
1: from org/jruby/RubyTime.java:1336:in `at'
RangeError (bignum too big to convert into `long')
```

Empieza reproduciendo el fallo en JRuby con `require "ssh_data"`; después inspecciona `lib/ssh_data/certificate.rb`, donde la carga llega actualmente a `Time.at`, y la ruta de carga relacionada en `lib/ssh_data.rb`. Identifica las otras ubicaciones que convierten 0 y 2^64-1 en marcas de tiempo y verifica que la biblioteca se cargue correctamente en JRuby sin el RangeError.

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