Change default precision for POST requests

[jmwerner]

Hello! This PR was inspired by losing decimal precision via a POST request, e.g.

> POST("http://httpbin.org/post", body = list(a = .8675309), encode = "json")
Response [http://httpbin.org/post]
  Date: 2017-10-18 17:46
  Status: 200
  Content-Type: application/json
  Size: 490 B
{
  "args": {}, 
  "data": "{\"a\":0.8675}", 
  "files": {}, 
  "form": {}, 
  "headers": {
    "Accept": "application/json, text/xml, application/xml, */*", 
    "Accept-Encoding": "gzip, deflate", 
    "Connection": "close", 
    "Content-Length": "12", 
...

This is happening because of the default jsonlite::toJSON behavior which can be easily fixed by passing a json to body in lieu of this PR if the dev team believes the default behavior should not be changed.

> POST("http://httpbin.org/post", body = jsonlite::toJSON(list(a = .8675309), digits = 10), encode = "json")
Response [http://httpbin.org/post]
  Date: 2017-10-18 17:46
  Status: 200
  Content-Type: application/json
  Size: 471 B
{
  "args": {}, 
  "data": "{\"a\":[0.8675309]}", 
  "files": {}, 
  "form": {}, 
  "headers": {
    "Accept": "application/json, text/xml, application/xml, */*", 
    "Accept-Encoding": "gzip, deflate", 
    "Connection": "close", 
    "Content-Length": "17", 
...

Please let me know if any changes are necessary. Thanks!

[hadley]

Thanks - this seems like a reasonable change to me.

Can you please add a bullet to NEWS? It should briefly describe the change and end with (@yourname, #issuenumber).

[hadley]

Thanks @jmwerner!