JSONRPC client does not set Content-Length header

[ryan-lang]

A fix similar to #796 is needed for the JSONRPC client, as it does not set the Content-Length header by default, which can cause issues decoding the content at the server. In my case, it's not even a 3rd party service - it's another go-kit JSONRPC service behind a load balancer.

I see three possible solutions:

1. something similar to transport/http: NewExplicitClient #971
2. set content-length by default. This could be accomplished with a single line: req.ContentLength = int64(b.Len()) immediately after the json is loaded into the request body
3. set content-length by hand in a before RequestFunc

Please advise on the preferred path, and if it's solution 1 or 2, I'd be happy to offer a PR.