Fixed bug caused by calling Poison.encode! on an empty request body

[stringlytyped]

In Togglex.json_request, the argument body is encoded to JSON using the Poison.encode! function. The default value for this argument is an empty string ("").

Calling Poison.encode! on an empty string returns "\"\"". In the past, Toggl's servers would process requests issued with this request body without issue. However, sometime within the last few months, the behavior of the API changed. Now when a request is submitted to Toggl with this value in the request body, Toggl returns an HTML page with an "invalid request" message.

This results in a :connect_timeout exception being raised (in version 0.2.0 of Togglex) when calling certain functions, such as Togglex.Reports.summary. (In version 0.1.0, a "Unexpected token" exception is raised.)

This pull request adds a check in the Togglex.json_request function that skips the call to Poison.encode! if body is empty.

[coveralls]

Coverage decreased (-6.5%) to 18.462% when pulling 5b91004 on stringlytyped:bug-fix/unexpected-token into 4b8d310 on diacode:master.

[stringlytyped]

Looks like the build is failing because String.trim used to be called String.strip in Elixir 1.2.0 (which Travis CI is configured to use when compiling).