The webdriver spec specifies that requests where the method is POST
should be parsed as JSON (see https://www.w3.org/TR/webdriver/#processing-model step 6).
Appium is too restrictive. It only parses as JSON if the application/json
Content-Type is provided, making it incompatible with some Webdriver clients
that follow the spec but don't send a Content-Type header.
This commit makes Appium less restrictive by defaulting to the application/json
Content-Type if none is provided.
Mykola Mokhnach