Mock network calls in test

[il3ven]

Closes #14

• I have used esmock along with fetch-mock. I could have skipped fetch-mock and mocked the transport.js module but then the tests would have lost their purpose.
• I have tried mocking network calls strictly. Instead of sending a response for any call I check for the method, URL and body.
• There are still network calls for the readme tests.

A brief explanation of the syntax.

const { default: getBlockNumber } = await esmock(
  "../src/blockNumber.js", // Module to be mocked
  null,
  {
   // Mock cross-fetch where ever it is imported
    "cross-fetch": {
      // This fetch-mock instance is the default export of cross-fetch
      default: fetchMock.sandbox().post(
        // Matcher object
        {
          url: options.url,
          body: {
            method: "eth_blockNumber",
          },
          matchPartialBody: true,
        },
        // Response object
        {
          result: "0xcd2057",
        }
      ),
    },
  }
);

[TimDaub]

Looks great! Thanks.

I'll merge once you have addressed the two superfluous console.logs in index.js