Consider renaming predicate JSON fields to snake case

[bartekn]

What version are you using?

1.9.0

What did you do?

Request claimable balance created by @abuiles: https://horizon-testnet.stellar.org/claimable_balances/0000000043d380c38a2f2cac46ab63674064c56fdce6b977fdef1a278ad50e1a7e6a5e18:

    {
      "destination": "GB6QDNU47MYBR4NDTRP7M3FW27DAFOEADN5KDQI2DAVWW6YVKKG4QJS7",
      "predicate": {
        "and": [
          {
            "not": {
              "absBefore": "2020-09-30T16:19:04Z"
            }
          },
          {
            "or": [
              {
                "absBefore": "2020-09-30T16:34:04Z"
              },
              {
                "not": {
                  "absBefore": "2020-10-01T22:17:24Z"
                }
              }
            ]
          }
        ]
      }
    }

What did you expect to see?

abs_before - snake case name consistent with other fields.

What did you see instead?

absBefore - camel case.

[abuiles]

Fixed in #3086