CVSS prefix is missing in v3.1.0

[woefe]

Subject of the issue

Version 3.1.0 produces invalid vector string. CVSS prefix is missing. From the spec:

The v3.0 vector string begins with the label "CVSS:"

Previous version (3.0.1) is not affected by this.

Your environment

• version of cvss-suite gem: 3.1.0
• version of ruby: 3.2.0

Steps to reproduce

irb(main):001:0> c = CvssSuite.new('CVSS:3.1/AV:L/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N/E:U/RL:O/RC:C')
=> 
#<CvssSuite::Cvss31:0x00007fb5ea2aede0
...
irb(main):002:0> c.vector
=> "AV:L/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N/E:U/RL:O/RC:C"

Expected behaviour

c.vector == 'CVSS:3.1/AV:L/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N/E:U/RL:O/RC:C'

Actual behaviour

c.vector == 'AV:L/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N/E:U/RL:O/RC:C'