Make CallRequest's to field optional

[HCastano]

According to the Ethereum Wiki page on JSON-RPC, the eth_call and eth_estimateGas calls use the same parameters, with the exception that all the fields for eth_estimateGas are optional.

Currently, CallRequest's to field was not optional. This PR changes that, making it suitable for use with eth_estimateGas.

I have a question about the RPC test I added with a None value for to. I didn't know what eth_estimateGas would return in that case, so I left the response from the test I copied. It passes, but I'm not sure it should considering that Value::String("0x123".into()) => 0x123 was in reference to the to address. What should this test check instead?

[tomusdrw]

looks good!

[tomusdrw]

 rpc_test! (
     /// What you want to call (with a test name)
      Eth:estimate_gas:optional_to_addr, CallRequest {
        from: None, to: None,
        gas: None, gas_price: None,
        value: Some(0x1.into()), data: None,
      }, None
      =>
     // what should be received (JSON)
      "eth_estimateGas", vec![r#"{"value":"0x1"}"#];
      /// What should the function return (`Value::String`) and what we expect to receive (JSON)
      Value::String("0x123".into()) => 0x123
    );

[tomusdrw]

The test is correct :)