RPC call returns wrong content-type

[keeperlink]

Describe the bug
RPC call returns wrong Content-Type header in response

To Reproduce
Steps to reproduce the behavior:

1. Run curl -v -X POST 'http://lotus:1234/rpc/v0' -H 'Content-Type: application/json' -H 'Accept: application/json' -d '{"jsonrpc": "2.0","method": "Filecoin.WalletBalance",
   "params": ["t1jdlfl73voaiblrvn2yfivvn5ifucwwv5f26nfza"],"id": 0}'
2. See in response "Content-Type: text/plain; charset=utf-8"

Expected behavior
Content-Type: application/json; charset=utf-8

Screenshots

curl -v -X POST 'http://lotus:1234/rpc/v0' -H 'Content-Type: application/json' -H 'Accept: application/json' -d '{"jsonrpc": "2.0","method": "Filecoin.WalletBalance",
"params": ["t1jdlfl73voaiblrvn2yfivvn5ifucwwv5f26nfza"],"id": 0}'

• About to connect() to 172.17.0.2 port 1234 (#0)
• Trying 172.17.0.2...
• Adding handle: conn: 0x56b39cb8
• Adding handle: send: 0
• Adding handle: recv: 0
• Curl_addHandleToPipeline: length: 1
• • Conn 0 (0x56b39cb8) send_pipe: 1, recv_pipe: 0
• Connected to 172.17.0.2 (172.17.0.2) port 1234 (#0)

POST /rpc/v0 HTTP/1.1
User-Agent: curl/7.30.0
Host: 172.17.0.2:1234
Content-Type: application/json
Accept: application/json
Content-Length: 117

• upload completely sent off: 117 out of 117 bytes
  < HTTP/1.1 200 OK
  < Date: Tue, 19 May 2020 15:00:26 GMT
  < Content-Length: 38
  < Content-Type: text/plain; charset=utf-8
  <
  {"jsonrpc":"2.0","result":"0","id":0}
• Connection #0 to host 172.17.0.2 left intact

Version (run lotus --version):
0.3.0'+git9a0d0511'+api0.3.0

Additional context

[whyrusleeping]

this seems like a bug in the rpc library. we should fix it.

[jennijuju]

should probably be transfer to go-jsonrpc repo