Filter insufficient source funds paths from pathfind results

[alandotcom]

When pathfinding with source amount, we need to filter out paths where source
amount is not equal to the specified source amount. This is due to the behavior
of rippled when specifying a source amount during pathfinding.

Example:

 {
  "command": "ripple_path_find",
  "source_account": "rhFQQ4ATC6MDF9ghTq3qAoCsGbGtjnhcXF",
  "destination_account": "rp91GUd5R3Rk3ipqW7XBdtrUJcX8epzGyb",
  "destination_amount": {
    "currency": "EUR",
    "issuer": "rp91GUd5R3Rk3ipqW7XBdtrUJcX8epzGyb",
    "value": -1
  },
  "send_max": {
    "currency": "USD",
    "issuer": "rhFQQ4ATC6MDF9ghTq3qAoCsGbGtjnhcXF",
    "value": "1234567891"
  },
  "id": 2
}
{
  "id": 2,
  "result": {
    "alternatives": [
      {
        "destination_amount": {
          "currency": "EUR",
          "issuer": "rp91GUd5R3Rk3ipqW7XBdtrUJcX8epzGyb",
          "value": "3999889.62127857"
        },
        "paths_canonical": [],
        "paths_computed": [
          [
            {
              "account": "rcsxQxEqU2qquAKp3tBUJy8Z2t19ioQPJ",
              "type": 1,
              "type_hex": "0000000000000001"
            },
            {
              "currency": "EUR",
              "issuer": "rp91GUd5R3Rk3ipqW7XBdtrUJcX8epzGyb",
              "type": 48,
              "type_hex": "0000000000000030"
            }
          ]
        ],
        "source_amount": {
          "currency": "USD",
          "issuer": "rhFQQ4ATC6MDF9ghTq3qAoCsGbGtjnhcXF",
          "value": "4170759.906037564"
        }
      }
    ],
    "destination_account": "rp91GUd5R3Rk3ipqW7XBdtrUJcX8epzGyb",
    "destination_amount": {
      "currency": "EUR",
      "issuer": "rp91GUd5R3Rk3ipqW7XBdtrUJcX8epzGyb",
      "value": "-1"
    },
    "destination_currencies": [
      "EUR",
      "XRP"
    ],
    "full_reply": true,
    "id": 2,
    "source_account": "rhFQQ4ATC6MDF9ghTq3qAoCsGbGtjnhcXF",
    "status": "success"
  },
  "status": "success",
  "type": "response"
}

[clark800]

LGTM