Merge rust-bitcoin#398: bitreq: Re-implement json-using-serde feature

99c948c Re-implement json-using-serde feature (Tobin C. Harding)
1251953 Replace stale docs reference to minreq (Tobin C. Harding)

Pull request description:

  This feature was removed when we gutted `minreq` after forking it. Turns out we need it to be able to use `bitreq` in `jsonrpc`.

  Re-implement the feature by copying code from `minreq`.

  Close: rust-bitcoin#393

ACKs for top commit:
  oleonardolima:
    tACK 99c948c
  jamillambert:
    ACK 99c948c

Tree-SHA512: 2e22881566fd3d75e0edb70567dac0b1ed9ce66720511dcead888b2b7686fc89fb1f6bce3bd490f91997e9ade88b0665b00679c79bcef35205c6ce1117306faf

Author: tcharding

bitreq/src/request.rs

@@ -297,7 +297,7 @@ impl Request {
     /// Returns `Err` if we run into an error while sending the
     /// request, or receiving/parsing the response. The specific error
     /// is described in the `Err`, and it can be any
-    /// [`minreq::Error`](enum.Error.html) except
+    /// [`bitreq::Error`](enum.Error.html) except
     /// [`InvalidUtf8InBody`](enum.Error.html#variant.InvalidUtf8InBody).
     #[cfg(feature = "async")]
     pub async fn send_async(self) -> Result<Response, Error> {