Updated the docs for rest_command (home-assistant#2430)

* Updated the docs for rest_command 

* Added the optional content_type config value, used to specify the request content type.

* Remove default content type of 'text/plain'

Should default to current behavior - no header

source/_components/rest_command.markdown

@@ -35,6 +35,7 @@ Configuration variables:
   - **username** (*Optional*): The username for HTTP authentication.
   - **password** (*Optional*): The password for HTTP authentication.
   - **timeout** (*Optional*): Timeout for requests. Defaults to 10 seconds.
+  - **content_type** (*Optional*): Content type for the request.
 
 The commands can be dynamic, using templates to insert values of other entities. Service call support variables for template stuff.