verification issue found with 'not' substring match

[lzx404243]

Noticed with Proxy Verifier version 2.12.0, with a - [fieldname, { value: [substr_to_match], not: contains }] directive, proxy verifier would fails verification whenever substr_to_match is longer than the value of fieldname. An example to demonstrate this:

      - client-request:
          ............
          headers:
            fields:
              # value is 7 bytes long.
              - [Cookie, "foo=bar"]

The following verification would fail:

        proxy-request:
          headers:
            fields:
              - [Cookie, { value: [12345678], not: contains }]

[INFO]: Not Contains Violation: Key: "demo", Field Name: "cookie", Required Values: "12345678", Received Values: "foo=bar"

while the following verification would pass:

  proxy-request:
          headers:
            fields:
              - [Cookie, { value: [1234567], not: contains }]

[INFO]: Not Contains Success: Not Found. Key: "invalid-acookie-not-sent-to-downstream-in-exempt-case", Field Name: "cookie", Required Values: "1234567", Received Values: "foo=bar"